What I say to you in darkness - That is, in "secret," in "private," in "confidence. The private instructions which I give you while with me do you proclaim publicly, on the "house-top." The "house-top," the flat roof, was a public, conspicuous place. See 2-Samuel 16:22. See also the notes at Matthew 9:1-8.

What I tell you in darkness - A man ought to preach that only which he has learned from God's Spirit, and his testimonies; but let him not pretend to bring forth any thing new, or mysterious. There is nothing that concerns our salvation that is newer than the new covenant; and in that there are, properly speaking, no mysteries: what was secret before is now made manifest in the Gospel of the ever-blessed God. See Ephesians 3:1-12.
What ye hear in the ear - The doctor who explained the law in Hebrew had an interpreter always by him, in whose ears he softly whispered what he said; this interpreter spoke aloud what had been thus whispered to him. Lightfoot has clearly proved this in his Horae Talmudicae, and to this custom our Lord here evidently alludes. The spirit of our Lord's direction appears to be this: whatever I speak to you is for the benefit of mankind, - keep nothing from them, declare explicitly the whole counsel of God; preach ye, (κηρυξατε proclaim), on the house-tops. The houses in Judea were flat-roofed, with a ballustrade round about, which were used for the purpose of taking the air, prayer, meditation, and it seems, from this place, for announcing things in the most public manner. As there are no bells among the Turks, a crier proclaims all times of public worship from the house-tops. Whoever will give himself the trouble to consult the following scriptures will find a variety of uses to which these housetops were assigned. Deuteronomy 22:8; Joshua 2:6; Judges 9:51; Nehemiah 8:16; 2-Samuel 11:2; 2-Kings 23:12; Isaiah 15:3; Jeremiah 32:29, and Acts 10:9.
Lightfoot thinks that this may be an allusion to that custom, when the minister of the synagogue, on the Sabbath eve, sounded with a trumpet six times, upon the roof of a very high house, that from thence all might have notice of the coming in of the Sabbath. The first blast signified that they should heave off their work in the field: the second that they should cease from theirs in the city: the third that they should light the Sabbath candle, etc.

What I tell you in darkness, [that] speak ye in light: and what ye hear in the ear, [that] preach ye upon the (m) housetops.
(m) Openly, and in the highest places. For the tops of their houses were made in such a way that they might walk upon them; (Acts 10:9).

What I tell you in darkness,.... Hence Christ proceeds to encourage his disciples to an open, plain, and faithful ministration of the Gospel, not fearing the faces and frowns of men. For with respect to the Gospel, his meaning is, that what was hid and covered should not remain so, but should be revealed, and made known, and they were the persons who were to do it; and it was with that view that he had communicated it to them: and whereas he had told them it "in darkness"; not in a dark and obscure manner; for though he spoke in parables to others, yet to them he made known the mysteries of the kingdom of heaven: and if at any time he delivered parables, or dark sayings, to them, he would afterwards, or when alone, explain them to them; but his meaning chiefly is, that what he communicated to them in private houses, when they were by themselves, and no one saw, or heard them, and so were in darkness with respect to others,
that speak ye in light; openly and publicly in the synagogues and temple, in the high places of the city, streets, or fields, wherever there is a concourse of people; hide and conceal nothing, but speak out all clearly, distinctly, fully, without the least reserve, or throwing any obscurity on it, which may cover the true sense of it from the view of the people.
And what ye hear in the ear, or is whispered to you by me, as your master. Christ alludes to the custom of the Jewish doctors, who had each an interpreter, into whose ear he used to whisper his doctrine, and then the interpreter delivered it to the people: so it is said (s),
"Rab came to the place of R. Shilla, and he had no speaker to stand by him; wherefore Rab stood by him, and explained.''
The gloss upon it is,
"an interpreter stands before a doctor whilst he is preaching, and the doctor , "whispers to him" in the Hebrew tongue, and he interprets it to the multitude in a tongue they understand.''
Again (t),
"they said to Judah bar Nachmani, the interpreter of Resh Lekish, stand for a speaker for him.''
The gloss upon it is,
"to cause his exposition to be heard by the congregation, , "which he shall whisper to thee".''
Now it was absolutely requisite, that the speaker, or interpreter, should faithfully relate what the doctor said; sometimes, it seems, he did not: it is said (u) in commendation of the meekness of R. Aba,
"that he delivered one sense, and his speaker said another, and he was not angry.''
The gloss says,
"his speaker was, he that interpreted to the multitude what he , "whispered to him" in the time of preaching.''
Sometimes one doctor is said to whisper in the ear of another, when he instructed him, or informed him of anything. R. Jochanan (w) whispered R. Joshua "in his ear". The Jews have a notion that the law was given this way; so they interpret "the eloquent orator" in Isaiah 3:3 (x) this is he to whom it is fit to deliver the words of the law, , "which was given by whispering": and so, it seems, the Gospel was in like manner delivered by Christ to his disciples. It was reckoned a very great honour, and a token of magisterial dignity, to have one to whisper in the ear to, and speak for them. So to one that related his dream, that he saw an ass standing at his pillow, and braying, answer is made, thou shalt be a king, that is, the head of a school; and "a speaker" or "an interpreter shall stand by thee" (y). Our Lord very justly takes upon him the character of a doctor, master, and dictator, and solemnly charges his disciples, clearly, loudly, and faithfully to declare what he suggested to them.
That preach ye, says he,
upon the housetops; for the roofs of their houses were not ridged, but plain, and flat, upon which they could stand or walk; and battlements were made about them to prevent their falling off, according to the law in Deuteronomy 22:8. Here many religious actions were performed: here Peter went up to pray, Acts 10:9 and here persons sometimes sat and read: hence that passage in the Misna (z) if any one , "was reading on the top of a roof", and the book is rolled out of his hand, &c. and sometimes they made their proclamations from hence of their festivals and solemn days, and particularly of their sabbath; which was done by the sound of a trumpet, that the people might cease from work in the fields, and shut up their shops in the city, and light up their lamps. This proclamation, by the sound of a trumpet, was made six times by the chazan, or minister of the congregation, from an housetop; and, it is said, that there was, , "a peculiar roof in the highest part of the city", and from the middle of it he blew the trumpet (a). In allusion to this, our Lord orders his disciples to blow the trumpet of the everlasting Gospel; and loudly proclaim to all the truths and mysteries of grace, which he had made known to them.
(s) T. Bab. Yoma, fol. 20. 2. (t) T. Bab. Sanhed. fol. 7. 2. (u) T. Bab. Sota, fol. 40. 1. (w) T. Hieros Kiddushin, fol. 65. 4. (x) T. Bab. Chagiga, fol. 14. 1. (y) T. Bab. Beracot, fol. 56. 1. (z) Erubin, c. 10. sect. 3. (a) T. Bab. Sabbat, fol. 35. 2. & Gloss. in ib.

What I tell you in darkness--in the privacy of a teaching for which men are not yet ripe.
that speak ye in the light--for when ye go forth all will be ready.
and what ye hear in the ear, that preach ye upon the housetops--Give free and fearless utterance to all that I have taught you while yet with you. Objection: But this may cost us our life? Answer: It may, but there their power ends:

What I tell you in darkness. In privacy. The Lord had to teach them in private before he could send them forth.
On the house tops. The flat roofs of eastern houses made a conspicuous pulpit. The Lord directs them to speak in the most public manner. In Syria proclamations are still often made from the house tops.

Even what I now tell you secretly is not to be kept secret long, but declared publicly. Therefore, What ye hear in the ear, publish on the house - top - Two customs of the Jews seem to be alluded to here. Their doctors used to whisper in the ear of their disciples what they were to pronounce aloud to others. And as their houses were low and flat roofed, they sometimes preached to the people from thence. Luke 12:3.