Deuteronomy 6:4

Here is, I. A brief summary of religion, containing the first principles of faith and obedience, Deu 6:4, Deu 6:5. These two verses the Jews reckon one of the choicest portions of scripture: they write it in their phylacteries, and think themselves not only obliged to say it at least twice every day, but very happy in being so obliged, having this saying among them, Blessed are we, who every morning and evening say, Hear, O Israel, the Lord our God is one Lord. But more blessed are we if we duly consider and improve, 1. What we are here taught to believe concerning God: that Jehovah our God is one Jehovah. (1.) That the God whom we serve is Jehovah, a Being infinitely and eternally perfect, self-existent, and self-sufficient. (2.) That he is the one only living and true God; he only is God, and he is but one. The firm belief of this self-evident truth would effectually arm them against all idolatry, which was introduced by that fundamental error, that there are gods many. It is past dispute that there is one God, and there is no other but he, Mar 12:32. Let us therefore have no other, nor desire to have any other. Some have thought there is here a plain intimation of the trinity of persons in the unity of the Godhead; for here is the name of God three times, and yet all declared to be one.

— Matthew Henry's Concise Commentary (public domain)