But thou (emphatic) when thou prayset, enter into thy closet, and when thou hast shut thy door, pray, etc. An adaptation of Isaiah 26:20 (cf. also 2 Kings 4:33). The prophet's language describing the action befitting a time of terror is used by our Lord to express what ought to be the normal practice of each of his followers.
Observe that the widow of one of the sons of the prophets so acted when she was about to receive the miraculous supply of oil (2 Kings 4:4, 2 Kings 4:5). Closet; Revised Version, inner chamber, more readily suggesting the passage in Isaiah to the English reader.
To thy Father which is in secret. Not "which seeth in secret," as in the next clause. The thought here may be partly that to be unseen of men is a help to communion with him who is also unseen by them, but especially that the manner of your actions ought to resemble that of your Father's, who is himself unseen and works unseen.
And thy Father which seeth in secret. You will be no loser, since his eyes pass by nothing, however well concealed it be from the eyes of men. Shall reward thee openly (verse 4, notes).