Theophylact of Ohrid

Elsewhere He says that the Father sends the Spirit; now He says He does: Whom I will send unto you; thus declaring the equality of the Father and the Son. That He might not be thought however to be opposed to the Father, and to be another and rival source, as it were, of the Spirit, He adds, From the Father; i. e. the Father agreeing, and taking an equal part in sending Him. When it is said that He proceedeth, do not understand His procession to be an external mission, such as is given to ministering spirits, but a certain peculiar, and distinct procession, such as is true of the Holy Spirit alone. To proceed is not the same as being sent, but is the essential nature of the Holy Ghost, as coming from the Father.