How must the Lord God be served?

Answer

We must serve God as he commands us in the Law, and Gospel (; ; ; ).

Read the 4 proof texts (KJV)

Numbers 15:39: And it shall be unto you for a fringe, that ye may look upon it, and remember all the commandments of the LORD, and do them; and that ye seek not after your own heart and your own eyes, after which ye use to go a whoring:
Jeremiah 7:31: And they have built the high places of Tophet, which is in the valley of the son of Hinnom, to burn their sons and their daughters in the fire; which I commanded them not, neither came it into my heart.
Matthew 28:20: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, [even] unto the end of the world. Amen.
John 14:15: If ye love me, keep my commandments.