Suppose somebody befriended you in your town and offered to buy you a big beautiful house, and told you that you could choose the house you wanted. Then when he was handing you the title deed you reached in your pocket and pulled out two pennys, and when you did they accidentally fell out of your hand in the mud, you then reach down and picked them up and put them in your friend's hand, You told him this house is too big of a gift, I must help pay for it. you'd be insulting him, and likewise we insult Christ when we try to add our filthy rags of righteousness to his precious gift of salvation, Jesus Paid It All and we can't add anything to that.

John 19:30 "When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost."