The idea is becoming popular once again that because of the fact that when Jesus died all our sins were still future, this means that all our future sins are already forgiven.
This can make people become totally unconcerned about whether they fall into sin or not. The reasoning is simple: "If my future sins are already forgiven, I am free to commit these sins, knowing that I will not be condemned because of them".
Some people go even further and say that because they are in Christ, God already considers them perfected regardless of their behavior, and that nothing they do could in fact possibly even COUNT as sin. This idea contradicts the New Testament Scriptures in plenty of places, but it is possible to twist certain Scriptures in order to come up with this doctrine – and people do.
Its not that everyone WOULD commit all the sins they could, or that they may be tempted to commit, when believing these thoughts. Some have a genuine love for Jesus even though they have certain false doctrines. Human beings are very complex and there is a whole array of factors that go into our core decision making process. However, because we still have internal programming that is contrary to God's will, and since there is a devil out there ready to present enticing and deceptive seductions to us, the thought that our future sins are already forgiven could encourage us to yield more easily to sin at certain times.
The Bible says: "If we confess our sins, He is faithful and just to forgive us our sins and cleanse us from all unrighteousness." (1 John 1:9). But if we believe that future sins are already forgiven, we'll also believe that there is no need to confess our sins in order to receive the cleansing and forgiveness this verse promises. That might be very dangerous. Its a case of allowing our human reasonings to overrider the clear teaching of Scripture. Since this is potentially a life or death issue, it is not something to casually play with.
The Bible says in 2 Peter chapter 1:
For this very reason, make every effort to add to your faith goodness; and to goodness, knowledge; 6 and to knowledge, self-control; and to self-control, perseverance; and to perseverance, godliness; 7 and to godliness, mutual affection; and to mutual affection, love. 8 For if you possess these qualities in increasing measure, they will keep you from being ineffective and unproductive in your knowledge of our Lord Jesus Christ. 9 But whoever does not have them is nearsighted and blind, forgetting that they have been cleansed from their past sins.
Why would Peter talk about "past sins" or "old sins" having been forgiven IF the REAL message of "unadulterated good news of God's grace" REALLY implies that such a person has also been cleansed of their FUTURE sins, including the sin of not being self-controlled, neglecting virtue, brotherly kindness, love and so on! It just doesn't make sense. Peter was communicating a different reality, a different message. That is why.