In Isaiah 6:8 God says, "Whom shall I send, and who will go for us?" God speaks of Himself on one hand as "I" and on the other hand as "us." This proves that "I" is "us" and "us" is "I"; "I" and "us" are identical. Then is God singular or plural? If you say He is plural, He says "I." If you say He is singular, He says "us." This is rather mysterious and difficult to understand; so we just take the scriptural revelation as it is.
Furthermore, in Genesis 1:26; 3:22; and 11:7, God also speaks of Himself as "us." The unique God, in His divine words, has many times spoken of Himself as "us." This is really a mystery difficult to comprehend. But we must believe that this is due to the matter of the three persons of the Godhead—the Father, Son, and Spirit.
The Lord says in Matthew 28:19, "Baptizing them into the name of the Father and of the Son and of the Holy Spirit." Here the Lord speaks clearly of the three persons—the Father, Son, and Spirit. But when He speaks here of the name of the Father, Son, and Spirit, the name which is used is in the singular number in the original text. This means that though the Father, the Son, and the Spirit are three, yet the name is one. It is really mysterious—one name for three persons. This, of course, is what is meant by the expression three-in-one, or triune.
Regarding "the name of the Father and of the Son and of the Holy Spirit," as spoken by the Lord here, we may ask, Is this name "Father," or "Son," or "Holy Spirit"? It is difficult to answer. All we can say is that the name of the Father, the Son, and the Holy Spirit is "Father, Son, and Holy Spirit." This name includes the three—Father, Son, and Holy Spirit—and tells us that God is three-in-one. Although God is only one, yet there is the matter of the three persons—the Father, the Son, and the Spirit.
In John 14:23 the Lord says, "If a man love me...my Father will love him, and we will come unto him, and make our abode with him." Also, in 17:11 He says, "Holy Father...that they may be one, even as we are." In both of these places the Lord speaks of Himself and the Father as "we." This must also be due to the matter of the Father, the Son, and the Holy Spirit.
(Concerning the Triune God—the Father, the Son, and the Spirit, Chapter 1, by Witness Lee)