What a great question! So many Christians struggle with this concept of abiding in Christ. I have some good news, you do not have to struggle any more! If you made a decision to receive Christ as your Savior, to believe He died on the cross for your sins and receive His forgiveness and grace, you are abiding right now! There is nothing left for you to do, the Father has already placed you in Christ...you are "abiding in Him". But wait, you may ask, doesn't Jesus tell us to abide? Yes, in John 15 he tells the listeners to abide in the vine and guess what, Jesus is the Vine (those who choose not to abide in Him are cast aside). Look, you are either IN CHRIST or not...you are either abiding (living) in Him, or not. Jesus simply used a wonderful illustration of the vine and branch to continue the same message He had from the beginning...trust Him, believe Him, receive Him, and so many more word pictures. You can't get any more "IN" than you are right now as a believer. The real question...are you experiencing the reality of your union with Christ? We can get distracted, disillusioned, depressed and more but that does NOT effect the reality and truth of who we are in Him. Stop trying to get "in" where you already "are"...it's like going to the front desk of a hotel and asking directions to get to that very same hotel...the front desk attendant will look at you oddly and declare, "but you are already here"? You ARE already here, now take some time to enjoy it. 

OK, this is a loaded question. The answer is, in my opinion, both. I won't get into the many, many years of theological debate re: predestination vs. free will (I've provided a few thoughts on the subject elsewhere). In general, we all have the God given ability to believe or have faith as a function of our will. Some would say our mind and will are tainted by sin and bend away from faith in Christ, thus God gives us His faith to believe (or chooses to make us righteous so we can believe thus the predestination angle). I take a slightly different approach…"And I, if I am lifted up from the earth, will draw all men to Myself" (John 12:32 NAS95S). I believe that God is drawing ALL men to Himself as a result of Christ's work on the cross. ALL men, however, must then choose to believe (and many will not). Some might argue that type of faith, as I have defined it, is a work of man and we all know "For by grace you have been saved through faith; and that not of yourselves, it is the gift of God; not as a result of works, so that no one may boast” (Eph 2:8-9 NAS95S). The sticking point here is that the word “gift” is not referencing "faith" but rather His grace and salvation. Salvation is not a work we do it is a gift of God that we receive by faith.

“being justified as a gift by His grace through the redemption which is in Christ. Jesus” (Rom 3:24 NAS95S)...note that the "justification" is the gift here as a result of the cross.

“For if by the transgression of the one, death reigned through the one, much more those who receive the abundance of grace and of the gift of righteousness will reign in life through the One, Jesus Christ.” (Rom 5:17 NAS95S)...note that "righteousness" is the gift that we "receive".

One other thought…God has also defined faith in the scriptures as choosing to believe. “But to the one who does not work, but believes in Him who justifies the ungodly, his faith is credited as righteousness” (Rom 4:5 NAS95S). "Believes in Him" is defined as "his faith" and further defined as "the one who does not work". Believing is not a work, it is called faith.

So, my faith or God's gift?…how 'bout BOTH

Absolutely not! Those who live by grace understand the truth of who they are in Christ (holy, pure and righteous)...sin is the opposite of their identity. You do not need a rule or a law to conform to your own identity...you are just you! And the "new you" created by God at salvation IS identified with the Lord Jesus Christ and IS by nature forgiven, righteous and accepted. Grace is not a license to sin but rather a license to LIVE as who you really are in Christ!

Repentance gets a lot of bad press. In my thinking it is grossly misused by the Church. The word simply means to think again, or consider after (reconsider); the context of scripture defines what we are to "re-think".

Many Christians attempt to define the word to mean an 180 degree turn from your sins...under the context of the believer, I might somewhat agree, but not for the unbeliever. The unbeliever does not turn from their sins to get saved, they simply reconsider the truth of the gospel and believe ((Mark 1:15 NASB) and saying, "The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel."

Salvation is a free gift of God, anything we do (including "turning from our sins") outside of receiving His gift of grace is attempting to merit His favor...that's not salvation. The bottom line: are you trusting in the finished work of Christ on the cross or something else for your salvation. Consider again or rethink what/who you're trusting in and trust Christ alone...that's repentance unto life eternal.

Check out an excellent theological article on the subject at:
http://www.bible.org/docs/theology/soter/wilkin/wilkin-03.htm#TopOfPage

First, the answer to the question is no. Second, the real issue is, "do believers still have the capacity to sin"? The answer is, ...yes. 

As a believer, our essential nature is "spirit". That spirit is either alive "in Christ" or dead "in Adam". To have 2 natures would mean a person possesses 2 spirits...quite an impossibility since your essential essence is spirit...in other words, when you die your spirit (singular) is separated from the body ((James 2:26 NASB) "For just as the body without the spirit is dead...."). You are a spirit being who lives on this earth in an earth suit we call a body...that spirit has life in Christ or is separated from life in Christ (i.e. death).

The capacity to sin still remains in the body (Romans 7 - this does not make the body evil, it simply means this body and our mind is susceptible to sin until we receive our heavenly bodies) and satan will use the old patterns of believing and behaving to deceive us into making wrong choices.

(2 Cor 6:16 NASB) Or what agreement has the temple of God with idols? For we are the temple of the living God; just as God said, "I WILL DWELL IN THEM AND WALK AMONG THEM; AND I WILL BE THEIR GOD, AND THEY SHALL BE MY PEOPLE.
The believer has the Lord Jesus Christ living on the inside (i.e. in their spirit). Since the Lord can not be joined to evil, it is not possible for the nature of the believer to include evil (1 Cor 6:17 NASB But the one who joins himself to the Lord is one spirit with Him. (i.e. we are baptized in the spirit. We are not God, but are joined together with Him)).

The only way you could lose your salvation would be if you earned it in the first place...and you didn't, you received the free gift of salvation by faith. The scripture says, "In Him, you also, after listening to the message of truth, the gospel of your salvation-- having also believed, you were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance, with a view to the redemption of God's own possession, to the praise of His glory (Eph 1:13-14 NASB.

“and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand.”
(John 10:28 NAS95)

Salvation is a one time event of you receiving His free gift of Life in Christ Jesus...it does not depend on your continual faithfulness (even in believing) but rather on His faithfulness. "If we are faithless, He remains faithful; for He cannot deny Himself." (2 Tim 2:13 NASB)

If we are really brand new creations at salvation with a new nature and Christ living on the inside...why do we still sin? Well, in the first place, many believers don't quite understand that first sentence. Some believers still see themselves as "sinners saved by grace"...well, what do sinners do? Right, they sin. So, many believers have a built in excuse for sin (although most would utterly deny ever excusing sin, it does become an "assumption" in their lives). My point is, many are not taught the truth of what really happened to them at salvation and thus never experience true victory. But what about those of us who know the truth of Christ yet still sin? 

Believers will never be out of the woods in this life when it comes to the possibility of sin...our whole life is nothing but a process of letting go of the old fleshly patterns we have come to rely on and allowing Christ to work in and through us in those very areas. Remember, God is perfecting you until the day of Christ (Philippians 1:6)...He wants you to experience His victory and Life more than you do!

Why do we still sin? Satanic strongholds that must be torn down; fleshly thinking that must be uncovered and replaced with truth; unmet needs that we continue striving to meet in our own strength. There could be many reasons.  Although this post is by no means the complete answer, the first step is to understand what happened at salvation and most importantly, to receive personally that God unconditionally loves and accepts you right now apart from any good or bad performance. Won't that just make it worse (i.e. "license to sin")? Actually, no, it's the first step to victory! Take some time to navigate through the site to find some more insights on this very big question!

Just let go and let God...right? Well kinda sorta. What that does NOT mean is a life of passivity! The "exchanged-life" or a life of grace can be very active...the difference?...activity does not define our life, Christ does. A life of grace is filled with all kinds of activities, choices, etc. where we place our full trust and dependence on Christ through the person of the Holy Spirit to not only energize and strengthen us, but actually perform through us His will and purpose. We move, choose, play and do trusting that the very life of Christ is working in and through us IN these very activities. We don't sit back and wait on God to physically carry our bodies...we go about our daily activity believing that He is the one performing...it's His strength...His resources...His Life.

So, "let go and let God" but get moving. Living by grace means I do not have to worry so much about results, my focus is on the process...i.e. my focus is on Christ as my Source and believing that He is actually working His will through me. How freeing...enjoy where you are and what you're doing cause it's His performance. The saying is so true, "it's not if you win or lose, it's how you play the game"...are you striving to perform or are you trusting Christ to perform through you. Winning or losing is not the point...how are you playing?

(Col 1:29 NASB) And for this purpose also I labor, striving according to His power, which mightily works within me.

A "stronghold" is simply a word picture describing the believer "caught" in a particular sin with seemingly no way of escape...the sin has a "strong hold" over them (it is spiritual bondage). Another way to understand this concept is when a particular sin or failure becomes a habit, grows to a compulsion that can end up as a full blown addiction. Often it's where your flesh tendencies intersect with satan's lies (1 Peter 5:8) that you believe or accept over time resulting in a powerful and sometimes seemingly irresistible pull toward sin. The foundation of just about every stronghold is "the lie". What happens? You believe and accept a lie about yourself, life, God or others and then, yield to some fleshly expression of that lie; absent the intervention of "truth", you succumb to the fleshly expression of that lie and develop/receive compounding lies that further strengthens the bondage ("it's just the way I am", "I can't help it", "it's no big deal", "it helps me cope" or other compounding lies). At this point, satan and his forces are masterfully controlling and influencing your life in this area (No, you're not possessed as a believer but you may sure act like it). Interestingly, as believers we often focus on the behavior and try to "fix" that stronghold (and there may be some societal value in that) yet the real strong hold to address is the lie(s). I would agree that often we need to stop the bleeding on the outside so we can have presence of mind to deal with the inside lies, but don't spend too much time outside. Attack the stronghold at it's foundation (the lie), resist the enemy by the authority you have in Christ (James 4:7), proclaiming the truth and trusting Christ for the victory in your behavior (Colossians 2:6). 

I think the word "possess" really bothers us. A believer can NOT be "possessed" by a demon because Jesus Christ has already taken up residence in their heart. I do think, however, that a believer can be so controlled by the enemy that you couldn't tell the difference whether they were possessed or not! If the real question is can a believer act just like the devil and be influenced by Satan and his demonic forces ...the unfortunate answer is yes.