Accepting the Divine Offer of Grace?

In the Christian story God becomes man and is crucified and through this saves us from our sin. But it's conditional upon something like accepting this gift. Is it enough to genuinely repent to God in the privacy of your own mind? Must you go on a campaign of trying to right the wrongs that you've done where you can? The divine offer of grace is not dependent upon that. Like until you do the last thing you don't get any grace. Rather it's a response to grace. It's having been grace with God's forgiveness of course and I want to make up for what I've done.