John 15:27

27.And you also bear testimony. Christ means that the testimony of the Spirit will not be of such a nature that the apostles shall have it for their private advantage, or that they alone shall enjoy it, but that by them it will be widely diffused, because they will be organs of the Holy Spirit, as indeed, he spoke by their mouth. We now see in what way faith is by hearing, (Romans 10:17,) and yet it derives its certainty from the seal and earnest of the Spirit, (Ephesians 1:13.) Those who do not sufficiently know the darkness of the human mind imagine that faith is formed naturally by hearing and preaching alone; (92) and there are many fanatics who disdain the outward preaching, and talk in lofty terms about secret revelations and inspirations, ( ἐνθουσιασμοὺς) But we see how Christ joins these two things together; and, therefore, though there is no faith till the Spirit of God seal our minds and hearts, still we must not go to seek visions or oracles in the clouds; but the word,

which is near us, in our mouth and heart,
(Romans 10:8,)

must keep all our senses bound and fixed on itself, as Isaiah says beautifully:

My Spirit that is upon thee, and my words which I have put in thy mouth, shall not depart out of thy mouth, nor out of the mouth of thy seed, nor out of the mouth of thy seed’s seed saith the Lord, from henceforth and for ever,
(Isaiah 59:21.)

Because you are with me from the beginning. This clause was added in order to inform us that so much the greater credit is due to the apostles on this ground, that they were eye-witnesses of what they relate; as John says,

what we have heard, what we have seen,
what our hands have handled, we declare to you;
(1 John 1:1)

for thus the Lord intended to provide for our welfare in every possible way, that nothing might be wanting for a full confirmation of the Gospel.