Ephesians 2:8-9 says" For it is by grace you have been saved, through faith - and this is not from yourselves, it is the gift of God - not by works, so that no one can boast."
The above is one of my favourite verses and I used to take great comfort in it. However, in our bible study last night I learned there is more to it. Now that we have been justified through faith, we now live in Christ and Christ lives in us.
Galatians 2:19-20: "For I through the law am dead to the law, that I might live unto God. I am crucified with Christ: nevertheless I live, yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me."
Hence, as Christians, although we are no longer condemned by the law, if we live as Christ lives in us, we will also be upholding God's law.
The key difference is this: living under the law, we actually live quite hypocritically, in the sense that we think all is good if we toe the line. However, living in the Spirit requires us to live for God. It is not about just toeing the line anymore, but what is in the heart is just as important. Just the thought of committing sin is as bad as having committed it.