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As per Luke 10::25 - 28 it was the Teacher of Law who on being prompted by our Lord [ with the question What do the Scriptures say? ] stated that the Greatest Commandment is to put it short : Love God and love your neighbour.
From where did the Teacher of Law get the reformulated Greatest Commandment vice the Ten Commandments? It should be there in the Old Testament itself; otherwise the Teacher of Law could not have given an answer to our Lord.
What we read in Deut. 6:5 is ''Love the God your Lord with all your heart with all your soul and with all your strength'' as the Commandment of God through words of Moses.. Again we read in Lev. 19:18 '' ------ love your neighbour as you as you love yourself''
The learned Teacher of Law had committed to his memory the above two quotations from the Old Testament. Our Lord is the embodiment of The Word of God said to him ''You are right''