Some ministers keep preaching the Old Covenant (Testament) to get Christians to go back and live under it, using it as a "cop-out".  Others will teach it right, saying, "This is what the Old Covenant states" ... and then they'll take us to the New Testament, saying, "This is what the New Covenant provides for us."   And this is the way it's supposed to be taught.  We do not live in the Old Covenant which God provided for the Jewish people before Jesus. 

For example,  the Old Testament states that God will fight your battles for you.  If this is correct, I have a couple questions to ask you. 

Why does Ephesians, chapter 6, tell us to put on the "armor of God"? ... why even have the "armor of God"?   Ephesians 6:11-17 tells us to put on this armor, what each piece means, and where it goes on your body. 

Battles were fought, and won or lost, with physical weapons in the Old Testament.  The full armor of God, on the other hand, is spiritual and is to be used against the devil and his works ... in the spirit realm.

Reverend Robert W. & Mary C. Butler