6 He proceeds to say, “And they have kept Your word: now they have known that all things, whatsoever You have given me, are of You;” that is, they have known that I am of You. For the Father gave all things at the very time when He begot Him who was to have all things. “For I have given unto them,” He says, “the words which You gave me; and they have received them;” that is, they have understood and kept hold of them. For the word is received when it is perceived by the mind.
“And they have known truly,” He adds, “that I came out from You, and they have believed that You sent me.” In this last clause we must also supply “truly;” for when He said, “They have known truly,” He intended its explanation by adding, “and they have believed.” That, therefore, “they have believed truly” which “they have known truly;” just as “I came out from You” is the same as “You sent me.” When, therefore, He said, “They have known truly,” lest any might suppose that such a knowledge was already acquired by sight, and not by faith, He subjoined the explanation, “And they have believed,” so that we should supply “truly,” and understand the saying, “They have known truly,” as equivalent to “They have believed truly:” not in the way which He intimated shortly before, when He said, “Do ye now believe?
The hour comes, and is now come, that you shall be scattered, every man to his own, and shall leave me alone.” But “they have believed truly,” that is, in the way it ought to be believed, without constraint, with firmness, constancy, and fortitude: no longer now to go to their own, and leave Christ alone. As yet, indeed, the disciples were not of the character He here describes in words of the past tense, as if they were so already, but as thereby declaring beforehand what sort they were yet to be, namely, when they had received the Holy Spirit, who, according to the promise, should teach them all things.
For how was it, before they received the Spirit, that they kept that word of His which He spoke regarding them, as if they had done so, when the chief of them thrice denied Him, after hearing from His lips the future fate of the man who denied Him before men? He had given them, therefore, as He said, the words which the Father gave Him; but when at length they received them spiritually, not in an outward way with their ears, but inwardly in their hearts, then they truly received them, for then they truly knew them; and they truly knew them, because they truly believed.
Source: Tractates on the Gospel of John (New Advent)