Charles Ellicott Commentary Deuteronomy 6:4-5

"Hear, O Israel: Jehovah our God is one Jehovah: and thou shalt love Jehovah thy God with all thy heart, and with all thy soul, and with all thy might." — Deuteronomy 6:4-5 (ASV)

Hear, O Israel ... —These two verses are called by our Lord the first and great commandment in the Law. The first words of the Talmud concern the hours when this form should be recited in daily morning or evening prayer—Hear, O Israel: Jehovah our God is one Jehovah. The unity of Jehovah, as opposed to the belief in gods many and lords many, is the keynote of the Jewish faith.

“We worship one God in Trinity, and Trinity in Unity.” But this truth, though visible in the Old Testament by the light of the New, was not explicitly revealed until it came forth in history, when the Father sent the Son to be the Savior of the world, and both sent the Holy Spirit to represent Him in the Church.