God is triune. Triune is from Latin. Tri means three, and une means one, so triune means three-one. Our God is three yet one and one yet three.
Elohim (God—Gen. 1:1) is plural, indicating that God is triune, three-one, as confirmed by the pronouns Us, Our, and He in Genesis 1:26-27; 3:22a; and 11:7.
Matthew 28:19 tells us that we baptize people into one name of three persons: into the name of the Father and of the Son and of the Holy Spirit. One name implies three persons, the Father, the Son, and the Spirit, indicating the Divine Trinity. Before the Lord spoke this word, nobody knew that God is the Father, the Son, and the Spirit. The Old Testament did point out in some verses that God is the Father, in other verses that God is the Son, and in some verses that God is the Spirit. The three were there in the Old Testament, but they were never put together as one until the time of Matthew 28:19.
Matthew 28:19 was spoken after Christ's resurrection. The Triune God was consummated in resurrection. Before resurrection the Triune God was in a process of being consummated. When God came into time, He entered into a process, and His process can be illustrated by a tunnel. That tunnel was from incarnation to resurrection. When He was resurrected, He came out of that tunnel, so resurrection is the consummation of His process. After the consummation of His process, the Lord Jesus revealed the Triune God. In the Gospels the Lord Jesus charged the disciples to go and baptize people into the Triune God, but in the Acts the disciples baptized people into the name of the Lord Jesus (Acts 8:16; 19:5). This indicates that the Lord Jesus equals the Father, the Son, and the Spirit—the Triune God.
The Father is the source of the Triune God, the Son is the expression of the Triune God (John 1:18), and the Spirit is the reaching, the application, of the Triune God (2 Cor. 13:14). These are simple, clear, and wonderful expressions. The Father is the fountain, the source, and the Son is the spring as the course to express the source. This course, this spring, issues in a flow, which is the Spirit as the reaching, the application, of the Triune God.