Acts 1:9

"And having this said, while they beheld, He was taken up; and a cloud received Him out of their sight." Seest thou that they did preach and fulfil the Gospel? For great was the gift He had bestowed on them. In the very place, He says, where ye are afraid, that is, in Jerusalem, there preach ye first, and afterwards unto the uttermost part of the earth. Then for assurance of what He had said, "While they beheld, He was taken up." Not "while they beheld" did He rise from the dead, but "while they beheld, He was taken up." Inasmuch, however, as the sight of their eyes even here was not all-sufficient; for in the Resurrection they saw the end, but not the beginning, and in the Ascension they saw the beginning, but not the end: because in the former it had been superfluous to have seen the beginning, the Lord Himself Who spake these things being present, and the sepulchre showing clearly that He is not there; but in the latter, they needed to be informed of the sequel by word of others.

But why did "a cloud receive Him?" This too was a sure sign that He went up to Heaven. Not fire, as in the case of Elijah, nor fiery chariot but "a cloud received Him;" which was a symbol of Heaven, as the Prophet says; "Who maketh the clouds His chariot" (Ps. civ. 3); it is of the Father Himself that this is said. Therefore he says, "on a cloud;" in the symbol, he would say, of the Divine power, for no other Power is seen to appear on a cloud. For hear again what another Prophet says: "The Lord sitteth upon a light cloud" (Is. xix. 1). For it was while they were listening with great attention to what He was saying, and this in answer to a very interesting question, and with their minds fully aroused and quite awake, that this thing took place. Also on the mount Sinai the cloud was because of Him; since Moses also entered into the darkness, but the cloud there was not because of Moses. And He did not merely say, "I go," lest they should again grieve, but He said, "I send the Spirit"; and that He was going away into heaven they saw with their eyes.

"He was taken up." Already it has been shown, that He went up into heaven. "And a cloud, etc." "Clouds and darkness are under His feet," (Ps. xviii. 9; xcvii. 2) saith the Scripture: for this is declared by the expression, "And a cloud received Him:" the Lord of heaven, it means. For as a king is shown by the royal chariot, so was the royal chariot sent for Him.