...answer continues from...

...The gospel is about the grace of God (Acts 20:24) and much more. 

"Grace for grace".in John 1:16 means, you have that which you believe in your heart that you have (Romans 11:6) and that determines your supply of the Holy Spirit. Jesus received the fulness of the Spirit.

Romans 6:15 says."What then? Shall we sin, because we are not under the law, but under grace? God forbid.".Here the words 'shall' and 'we' are not in the original Greek. 

In keeping with the tenor of the book (Romans 4:15), a more accurate translation from the original Greek of Romans 6:15 would have been:."Is there sin if no law exists? No!"

Paul is explaining "Is it possible to now sin that we are no longer under the law but under grace?" 

In the previous verse (Romans 6:14) Paul says sin has no dominion over one (word 'shall' not in original).

Romans 6:13: Paul is saying that is why you think that you are still sinning by what you do? 

When one tries to get himself right in order to be acceptable to God, apart from faith righteousness (the righteousness given as a gift from God because one believes in Jesus Christ), then he has fallen from grace.

Through faith and love, the grace of God is abundant for you:.1Timothy 1:14. How to get it! 

Paul, throughout his writings in the New Testament, encouraged spiritual people to stay in the faith. Some were getting into gospel perversion. Perversion is mixing two things together that do not go together. See 'Alternate Gospel'.