Abraham was a Christian!
God never gave old Abe the Law!
Gal_3:8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
So if you do not understand the Scripture verse above let me explain it to you!
The Bible consists of 2 Testaments, the Old Testament and the New Testament.
What the verse above is teaching is the Abraham followed the New Testament teachings!
God gave the Children of Israel the Old Testament which was NOT the one He gave Abraham!
Deu 5:2 The LORD our God made a covenant (The Old covenant) with us in Horeb.
Deu 5:3 The LORD made not this covenant (The Old Covenant) with our fathers (Abraham, Isaac, Jacob), but with us, even us, who are all of us here alive this day.
The days are coming where Once again the Jews will accept the New Covenant!
Jer 31:31 Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:
Jer 31:32 Not according to the covenant (The Old Covenant) that I made with their fathers (in Moses’ days) in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was an husband unto them, saith the LORD:
Jer 31:33 But this shall be the covenant (The New Covenant) that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.
Jer 31:34 And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.
So stick with the New Covenant and live by Faith in Jesus Christ like Abraham did!
Gal 3:7 Know ye therefore that they which are of faith, the same are the children of Abraham.