John 15:26

26.But when the Comforter is come. After having explained to the apostles that the Gospel ought not to be less highly valued by them, because it has many adversaries, even within the Church itself; Christ now, in opposition to the wicked fury of those men, produces the testimony of the Spirit, and if their consciences rest on this testimony, they will never be shaken; as if he had said, “True, the world will rage against you; some will mock, and others will curse your doctrine; but none of their attacks will be so violent as to shake the firmness of your faith, when the Holy Spirit shall have been given to you to establish you by his testimony.” And, indeed, when the world rages on all sides, our only protection is, that the truth of God, scaled by the Holy Spirit on our hearts, despises and defies all that is in the world; for, if it were subject to the opinions of men, our faith would be overwhelmed a hundred times in a day.

We ought, therefore, to observe carefully in what manner we ought to remain firm among so many storms. It is because

we have received, not the spirit of the world, but the Spirit which is of God, that we may know the things which have been given to us by God,
(1 Corinthians 2:12.)

This single witness powerfully drives away, scatters, and overturns, all that the world rears up to obscure or crush the truth of God. All who are endued with this Spirit are so far from being in danger of falling into despondency on account of the hatred or contempt of the world, that every one of them will obtain glorious victory over the whole world. Yet we must beware of relying on the good opinion of men; for so long as faith shall wonder in this manner, or rather, as soon as it shall have gone out of the sanctuary of God, it must become involved in miserable uncertainty. It must, therefore, be brought back to the inward and secret testimony of the Spirit, which, believers know, has been given to them from heaven.

The Spirit is said to testify of Christ, because he retains and fixes our faith on him alone, that we may not seek elsewhere any part of our salvation. He calls him also the Comforter, that, relying on his protection, we may never be alarmed; for by this title Christ intended to fortify our faith, that it may not yield to any temptations. When he calls him the Spirit of truth, we must apply the term to the matter in hand; for we must presuppose a contrast to this effect, that, when they have not this Witness, men are carried about in various ways, and have no firm resting-place, but, wherever he speaks, he delivers the minds of men from all doubt and fear of being deceived.

When he says that he will send him from the Father, and, again, that he proceedeth from the Father, he does so in order to increase the weight of his authority; for the testimony of the Spirit would not be sufficient against attacks so powerful, and against efforts so numerous and fierce, if we were not convinced that he proceedeth from God So then it is Christ who sends the Spirit, but it is from the heavenly glory, that we may know that it is not a gift of men, but a sure pledge of Divine grace. Hence it appears how idle was the subtlety of the Greeks, when they argued, on the ground of these words, that the Spirit does notproceed from the Son; for here Christ, according to his custom, mentions the Father in order to raise our eyes to the contemplation of his Divinity.