John Lightfoot on John 14:7

[If ye had known me, &c.] It was a very difficult thing to spell out the knowledge of the Messiah from the law and the prophets under the first Temple; but it was doubly more difficult under the second. For, under the first Temple, Moses had only his own veil over him, and the prophets only their own proper and original obscurity: but under the second Temple, the obscurity is doubled by the darkness and smoke of traditions; which had not only beclouded the true doctrines of faith and religion, but had also brought in other doctrines diametrically contrary to the chief and principal articles of faith: those for instance concerning justification, the person, reign, and office of the Messiah, &c.

With what measures of darkness these mists of tradition had covered the minds of the apostles, it is both difficult, and might be presumptuous, to determine. They did indeed own Jesus for the true Messiah, John 1:41; Matthew 16:16: but if in some things they judged amiss concerning his office, undertaking, and government, we must put it upon the score of that epidemical distemper of the whole nation which they still did in some measure labour under. And to this may this clause have some reference, "If ye had known me, and had judged aright concerning the office, undertaking, and authority of the Messiah, ye would, in all these things which I teach and do, have known the will, command, and authority of the Father."

[And from henceforth ye know him.] We may render it, Henceforward therefore know him: "Henceforward acknowledge the Father in all that I have done, brought in, and am to introduce still, and set your hearts to rest in it: believing that you see the Father in me, and in the things that I do."