The effort to repay God, in the ordinary way we pay creditors, would nullify grace and turn it into a business transaction. If we see acts of obedience as installment payments, we make grace into a mortgage... Let us not say that grace creates debts; let us say that grace pays debts. - John Piper Future Grace
What Christ did for me is completely free. This new life in Him is free, and totally 100% initiated by Him and through Him.
What generates or motivates (if you will) obedience to God?
Past grace is glorified by intense and joyful gratitude. Future grace is glorified by intense and joyful confidence. This faith is what empowers us for venturesome obedience in the cause of Christ. - John Piper Future Grace
Gratitude for what Christ has done for me in the past is not the primary motivator for my obedience to Him. However, gratitude glorifies past grace from God. If gratitude is my reason to obey Christ in the future, am I not inadvertently trying to repay God? "You should obey God because look at what He's done for you." I have heard these words come out of my mouth when my kids ask why we should obey. Albeit the doctrine of grace is a little difficult even for me to understand, I don't want to instill the need or requirement to somehow repay God. It's heretical, it's wrong, it's bondage. I know in my head that I can't repay God. But this was profound for me. It is causing a shift in the way I talk about obedience with my kids, and the way I think about how I am walking in Christ.
We are free to obey God because of the grace He will give us.