Enter Deuteronomy 6:20-27. God tells Moses how to address His sacred legacy, Grace and Truth. From the paraphrased version The Message (in italics):
The next time your child asks you, "What do these requirements and regulations and rules that God, our God, has commanded mean?"
GG: When my kids ask why I believe what I believe, why I want to obey God...
tell your child, "We were slaves to Pharaoh in Egypt and God powerfully intervened and got us out of that country.
GG: I remind them of my initial state and what God's Grace did for me...
We stood there and watched as God delivered miracle-signs, great wonders, and evil-visitations on Egypt, on Pharaoh and his household.
GG: I did nothing to earn His miraculous and wonderful Grace. God did it all, and it was beautiful...
He pulled us out of there so he could bring us here and give us the land he so solemnly promised to our ancestors.
GG: I remind myself where I was and to where I have been led, as wel as how He is faithful to His promises...
That's why God commanded us to follow all these rules, so that we would live reverently before God, our God, as he gives us this good life, keeping us alive for a long time to come. "It will be a set-right and put-together life for us if we make sure that we do this entire commandment in the Presence of God, our God, just as he commanded us to do."
GG: And because of the inexplicable Grace God has given me, I will obey Him, live reverently before Him, thankful of the good life He has provided for me.
It's Grace. And that, kids, is why I obey God's Truth.