The only reason God became a man is so he could live and die in our stead.God simply could not Die unless he became a mortal man.And a mortal man could not take his life up again unless he was the true God .and what would be the sence of dying for sin ful mans stead Unless God lived man's life perfectly for them first than dying for them would win them heaven. no matter how we want to thank God for all the above we just can't live a sinless life.Thats why God justifies us or better said declaires we are right with him because of Jesus.Gods decloration does not change the fact that we still are sinners What it changes is that God because of what Jesus has done for us . now accepts us the sinner as if we had no sin. All for the sake of his Son who lived life perfect for us and died for our sin's winning us heaven by His Good and faith ful work not ours works that are always tainted with sin .be it by thought or unperfect motives words or deed .