3. Know that He is one

The third commandment in Mosaic Law is to know that He is One.

Where in scripture?

Deuteronomy  6:4

 "Hear, O Israel! The LORD is our God, the LORD alone!"

New Testament references

1 Corinthians 8:6 

"…yet for us there is one God, the Father, from whom all things are and for whom we exist, and one Lord, Jesus Christ, through whom all things are and through whom we exist." 

2 Corinthians 13:13

The grace of the Lord Jesus Christ and the love of God and the fellowship of the holy Spirit be with all of you.

1 Timothy 2:5-6

For there is one God. There is also one mediator between God and the human race, Christ Jesus, himself human, who gave himself as ransom for all."

Matthew 28:19

"Go, therefore, and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit"

Catechism of the Catholic Church

234 The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the "hierarchy of the truths of faith". The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son and Holy Spirit, reveals himself to men "and reconciles and unites with himself those who turn away from sin".

237 The Trinity is a mystery of faith in the strict sense, one of the "mysteries that are hidden in God, which can never be known unless they are revealed by God". To be sure, God has left traces of his Trinitarian being in his work of creation and in his Revelation throughout the Old Testament. But his inmost Being as Holy Trinity is a mystery that is inaccessible to reason alone or even to Israel's faith before the Incarnation of God's Son and the sending of the Holy Spirit.

242 Following this apostolic tradition, the Church confessed at the first ecumenical council at Nicaea (325) that the Son is "consubstantial" with the Father, that is, one only God with him. The second ecumenical council, held at Constantinople in 381, kept this expression in its formulation of the Nicene Creed and confessed "the only-begotten Son of God, eternally begotten of the Father, light from light, true God from true God, begotten not made, consubstantial with the Father".

245 The apostolic faith concerning the Spirit was confessed by the second ecumenical council at Constantinople (381): "We believe in the Holy Spirit, the Lord and giver of life, who proceeds from the Father." By this confession, the Church recognizes the Father as "the source and origin of the whole divinity". But the eternal origin of the Spirit is not unconnected with the Son's origin: "The Holy Spirit, the third person of the Trinity, is God, one and equal with the Father and the Son, of the same substance and also of the same nature. . . Yet he is not called the Spirit of the Father alone,. . . but the Spirit of both the Father and the Son." The Creed of the Church from the Council of Constantinople confesses: "With the Father and the Son, he is worshipped and glorified."

253 The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity". The divine persons do not share the one divinity among themselves but each of them is God whole and entire: "The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God." In the words of the Fourth Lateran Council (1215), "Each of the persons is that supreme reality, viz., the divine substance, essence or nature."

261 The mystery of the Most Holy Trinity is the central mystery of the Christian faith and of Christian life. God alone can make it known to us by revealing himself as Father, Son and Holy Spirit.

262 The Incarnation of God's Son reveals that God is the eternal Father and that the Son is consubstantial with the Father, which means that, in the Father and with the Father the Son is one and the same God.

266 "Now this is the Catholic faith: We worship one God in the Trinity and the Trinity in unity, without either confusing the persons or dividing the substance; for the person of the Father is one, the Son's is another, the Holy Spirit's another; but the Godhead of the Father, Son and Holy Spirit is one, their glory equal, their majesty coeternal" (Athanasian Creed: DS 75; ND 16).

Early Church Fathers

St. Augustine, De Trin. 15, 26, 47: PL 42, 1095

"The Holy Spirit proceeds from the Father as the first principle and, by the eternal gift of this to the Son, from the communion of both the Father and the Son."

St. Gregory of Nazianzus, Oratio 40,41: PG 36,417

"'O blessed light, O Trinity and first Unity!' God is eternal blessedness, undying life, unfading light. God is love: Father, Son and Holy Spirit… I give you but one divinity and power, existing one in three, and containing the three in a distinct way."

Conclusion

Binding on Christians