John Gill

While he thus spake there came a cloud,.... While Peter was making the above request, before an answer was returned, a cloud appeared, a very uncommon one, as a symbol of the divine presence: "and overshadowed them"; Jesus, Moses, Elias, and the disciples: and they feared as they entered into the cloud; either as they themselves entered into it, that coming gradually over them, because of the glory of it, and the solemnity that attended it; or as Moses and Elias entered into it; and so the Syriac and Persic versions read, "they feared when they saw Moses and Elias enter into the cloud"; which took them out of their sight: just as the cloud received Jesus out of the sight of his disciples, when he ascended to heaven, Act 1:9.