John Chrysostom

Then, having rid them of all distress, and fears, and anxiety, and set them above men's reproaches, then, and not till then, He seasonably discourses to them also of boldness in their preaching. For, "What I tell you," saith He, "in darkness, that speak ye in light; and what ye have heard in the ear, that preach ye upon the housetops." Yet it was not at all darkness, when He was saying these things; neither was He discoursing unto them in the ear; but He used a strong figure, thus speaking. …

Homily on the Gospel of Matthew 34 ↗

Theophylact of Ohrid

"What I tell you in darkness, that speak ye in light: and what ye hear in the ear, that preach ye upon the housetops." Those things, He says, which I have spoken to you alone and in one place, for that is what is meant by "in the ear" and "in darkness" you must teach with boldness in a strong voice so that all may hear you. But since dangers are also a consequence of boldness, He adds:

Commentary on Matthew ↗

Jerome

We do not read that the Lord was wont to discourse to them by night, or to deliver his doctrine in the dark; but He said this because all His discourse is dark to the carnal, and His word night to the unbelieving. What had been spoken by Him they were to deliver again with the confidence of faith and confession. Otherwise; What you hear in mystery, that teach in plainness of speech; what I have taught you in a corner of Judæa, that proclaim boldly in all quarters of the world.

Catena Aurea by Aquinas ↗

Hilary of Poitiers

We do not take this to mean that the Lord was accustomed to preach at night and to transmit his teaching in the dark. Rather, to those who are carnal, everything he said is darkness, and to unbelievers his word is night.… Therefore Jesus commands that those things which were said in the darkness should be preached in the light. In this way the things he secretly whispered into their ears will be heard from the housetops, from on high, through those who speak as heralds.

Commentary on Matthew 10.17 ↗

Thomas Aquinas

Then the Lord instructs them as an advocate is instructed, because he is first taught how to present proof, before making it known to others. Thus, the Lord had chosen disciples to spread his word to all people; therefore, he chose to teach them first in secret, saying, what I tell you in the darkness, utter in the light. There are two senses involved in a disciple's learning, namely, hearing and sight. What is said in the darkness is hidden; in like manner, what is heard by the ear. …

Commentary on Matthew ↗