"Why do some Christians teach that even though Jesus took our sins away from the sight of God, we still need to repent for them? Does not the bible say that Jesus took our sins away forever, past, present, and future...?", (Question from Manny Ferreira).
Repentance is a big part of the salvation process, because as we approach the throne of God, our own faults and culpabilities are made apparent to us, "Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel, and forgiveness of sins" (Luke 24.47 KJV bible). When we come to recognize and repent for our sins, Christ washes them away, making us new creatures in his name. But then after we are redeemed in Christ, should repentance continue to be a part of our daily lives? The answer is yes and no.
First, we should not seek forgiveness for things we've already repented for, because we are no longer in bondage to these "dead works", "Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God" (Hebrews 6.1 KJV bible). Paul refers to the sins of our old self as dead works, because these sins are forgiven, and no longer relevant to our lives. He instructs us to move on past repentance from these old sins, and unto the meat of God's Word. This doesn't mean that we shouldn't remember, or think about, or understand these dead works, but that we shouldn't ask to be forgiven from them again and again. Re-repentance from dead works is an insult to Christ, because it implies that initially he did not fully heal us, and set us free from our sins.
On the other hand, as Christians we must always strive to maintain a humble and repentant disposition in Christ. The reality is that we are not perfect, and we will never be perfect. Only in Christ are we perfected to be received by God as his children (Ephesians 1). As fallible people who inevitably sin, it's our responsibility to resist and war against sin to prevent it from further manifesting in our lives. In Revelation, Jesus instructs the church of Sardis to repent from its sins, and turn back to him, "Remember therefore how thou hast received and heard, and hold fast, and repent. If therefore thou shalt not watch, I will come on thee as a thief, and thou shalt not know what hour I will come upon thee" (Revelation 3.3 KJV bible). Many at Sardis had become derelict and fallen away from the righteous path of Christ. So Jesus' rebuke serves as a warning for all Christians to discern and resist sin before it overtakes us.
Christ gives us eyes that see, so we can recognize our sins, and improve and work on ourselves. This striving to become more Christlike is a responsibility that we have as part of our relationship with him, and not a gift like salvation, "For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God: But that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned" (Hebrews 6.7-8 KJV bible). In Christ, we are forgiven from our sins, but if we refuse to recognize and confess our own sins and failings, we have rendered ourselves unusable for his purpose and plan.
Now this doesn't mean that the second a Christian sins they are out of the book, and no longer a Christian. Our inevitable sins our washed clean if we believe in and follow Christ. What is does mean is that it's not okay for us to sin, so we have to war against it in Christ, "What shall we say then? Shall we continue in sin, that grace may abound? God forbid. How shall we, that are dead to sin, live any longer therein?" (Romans 6.1-2 KJV bible). So it is the willful and habitual sinner, living in sin, whom the wrath of God will be upon when Christ returns to reign (Ephesians 5.3-6).