sin is as put in the bible, transgression of the law, since we are not under the law anymore but under the spirit it would be transgression of the spirit, which is the spirit of god which would be going against god, the only way to not go against him is to fully submit ourselves to him. i also beleive sin is necessary, we must sin and come back to god, the bible states that god is more joyed by a man who sins and repents (not just repents but bears fruit in keeping with repentance) then someone who has never needed forgiveness, it is a part of our ongoing creation and the building of our character. mans fall was intentional, saying it was not makes god fallible, which he is not, think long and hard and prayerfully read the bible and you will see what i mean.