In a previous message, we saw that the Son came in the Father's name (John 5:43a) and that the Son did things in the name of the Father (John 10:25). John 14:26 says that the Father sends the Spirit in the Son's name. The First (the Father) sends the Third (the Spirit) in the Second's (the Son's) name. The Father does not do things by Himself alone. He does things in the Son's name.
To do things in the name of another one is not only to be one with another person but also to be that other person. The Father sent the Spirit in the Son's name. This is like writing a check but signing the name of another person, instead of your own name. Usually, the person who writes the check signs his own name. But if I write a check and sign another brother's name, it indicates, at least in part, that the other brother wrote the check. The Father "wrote the check" by sending the Spirit, but He "signed" the name of the Son by sending the Spirit in the name of the Son. The Father, as the Son, sent the Spirit.
In John 14:26 the Father sends the Spirit in the name of the Son, but in John 15:26, the Son sends the Spirit from the Father. When these two portions are put together, you can see that when the Father sends the Spirit, He does so with the Son. In the same manner, the Son sends the Spirit with the Father. Neither the Father nor the Son would do anything without the other. When the Father sends the Spirit in the Son's name, He does it as the Son. When the Son sends the Spirit from the Father, He does it as one with the Father. Then when the Spirit comes, He testifies concerning the Son (15:26). This shows the oneness of the wonderful divine Trinity.
The Son was made the Firstborn of all creation (Col. 1:15) and the Firstborn in resurrection (v. 18). In making the Son the Firstborn of the two creations, the Father gave the preeminence, the first place, to the Son. Colossians 1:19 says, "For in Him all the fullness was pleased to dwell." The fullness mentioned in this verse is the fullness of the Godhead (2:9). This means that all the fullness of the Godhead was pleased to dwell in Christ, who is God's embodiment. The Father, Son, and Spirit were pleased to dwell in Christ. As the embodiment of the divine Trinity, the Son did not "regard equality with God a thing to be grasped, but emptied Himself, taking the form of a slave, becoming in the likeness of men" (Phil. 2:6-7). The Son lowered Himself to be a slave, taking the form of man. He was happy to obey God, even to the point of death—not an ordinary death, but the death of the cross (v. 8). The Son was willing to be lowered and to obey the Father to such an extent. The Father honored the Son, the Son lowered Himself to obey the Father, and the Spirit, being sent by the Father and the Son, came to testify concerning the Son. While He was on earth, the Son did nothing of Himself (John 5:19, 30). Rather, He did everything by the Father according to the Father's will. The three Persons of the Godhead are mingled as one. The church's oneness should be a duplication of the divine Trinity's oneness.