The Bible tells us that God commands us to be holy as He is holy. If we were allowed to sin and still make it into heaven, there would be no point to having a heaven or hell. Why would "be holy as I am holy" even be in the Bible if it was not something that God wanted us to do. In the scripture it says to lay aside every weight. Why would God tell us to lay aside every weight if He really didn't mean every weight. The Scripture says wisdom, which is applied knowledge, is calling aloud, meaning if applied to this that God is not being cryptic. Holiness is the standard. How are we as Christians supposed to stand before God in his kingdom with the likes of Steven, Job and David, if it's okay for us to sin? Yes, David committed sin with Bathsheba, but he repented continually and did not repeat his sin. Yes, God offers forgiveness, but only those who truly feel godly sorrow and wish to repent and not repeat will be forgiven. The Scripture says shall we continue to sin that grace may abound, God forbid. I really don't understand what you're talking about when you say the concept that a Christian cannot sin is false because if you were truly a Christian you would not want to sin. The worldy assumption is that no one is perfect, so God wouldn't require us to be perfect. But we can in fact be perfect because we can do all things through Christ Jesus who strengthens us, so we do not operate under the assumptions and theories of the world because we are called to be a peculiar people (I Peter 1:16; Hebrews 12:1; Proverbs 1:20; Romans 6:1-7; Psalms 51:3; I Peter 2:9).
"This is the message which we have heard from Him and declare to you, that God is light and in Him is no darkness at all. If we say that we have fellowship with Him, and walk in darkness, we lie and do not practice the truth. But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin. If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His word is not in us. My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous" (1 John 1:5 - 2:1).
No one has stated that sin is acceptable, but that sin happens to the best Christians. Nor do you find that a forgiven Christian will not ever repeat an old sin. It shouldn't happen, but there nothing stating that it cannot happen. "Take heed to yourselves. If your brother sins against you, rebuke him; and if he repents, forgive him. And if he sins against you seven times in a day, and seven times in a day returns to you, saying, 'I repent,' you shall forgive him" (Luke 17:3-4). There is a difference between what we strive for and what we manage to accomplish.