John 15:26

But when the Comforter is come
Or advocate, the Spirit of God; who was to be, and has been an advocate for Christ, against the world, and for his people, against all their enemies; and who as he was to reprove, and did reprove the world of sin, righteousness, and judgment, in favour of Christ, so he was to assist his people, and plead their cause, and help them, in vindication of themselves, before the princes of the earth, as he did: and who also was to act, and has acted the part of a "comforter" to them, under all the hatred and violence they have met with from the world; by taking and applying the things of Christ to them; by shedding the love of God in them; by applying the promises of the Gospel to them; by witnessing their adoption, and sealing them up to the day of redemption:

whom I will send unto you from the Father;
visibly, as on the day of Pentecost, in cloven tongues as of fire; and invisibly into their hearts, by the secret influence of his light and grace; which mission, as it suggests no inferiority in the spirit, either to the Father or the Son; since the same spirit with the Father, was the sender of Christ; so it is expressive of the equal deity of Christ, and his joint power and authority with the Father:

even the Spirit of truth;
who is the true Spirit, truth itself; yea, the true God, with the Father and Son; the Spirit of him who is truth; the dictator of the Scriptures of truth; who leads his people into all truth; and is the Spirit of truth, as he is a witness or testifier of Christ, hereafter promised:

which proceedeth from the Father;
Christ is not content to describe him by his work and office, as, an, advocate and comforter, and as the Spirit of truth: and from his mission by him from the Father; all which shows his usefulness and authority; but also from his nature and essence, which is the same with the Father's; and from his peculiar personal and distinctive character, expressed by his proceeding from the Father; and which is mentioned, as what is distinct from his mission by Christ, from the Father before spoken of; and designs no other, than the eternal, ineffable, and continued act of his procession, from the Father and the Son; in which he partakes of the same nature with them, and which personally distinguishes him from them. The ancient Jews ^F24 spoke of him just in the same language; "the Spirit of God", in ( Genesis 1:2 ) ; they say is the Holy Spirit, (Myhlam) (qypnd) , "which proceedeth from God": very pertinently does Christ take notice of this his character here, when he was about to speak of him as his testifier:

he shall testify of me:
of his deity and sonship, of his incarnation, of his being the Messiah, of his sufferings and death, of his resurrection and ascension, of his exaltation at the right hand of God, and of his ordination to be the Judge of quick and dead; all which he bore testimony to, by the gifts bestowed upon the apostles, and the great grace that was upon them all; by the signs, wonders, and divers miracles, by which the Gospel of Christ was confirmed; and by the power, influence, and success, which attended the preaching of it every where. Thus he testified of Christ, against the blaspheming Jews, and persecuting Gentiles, to the reproof and confusion of them; and he testified of him to the apostles, and all true believers, to their great joy and comfort, and to the support of them, under all the malice and hatred of the world.