Theophylact of Ohrid

Since they were simple and unlearned men, lest they be troubled that wise men would demand an account from them, He says that they should not be concerned about this at all.

Commentary on Luke ↗

Cyprian

For that it is His doing that we conquer, and that we attain by the subduing of the adversary to the palm of the greatest contest, the Lord declares and teaches in His Gospel, saying, "But when they deliver you up, take no thought how or what ye shall speak; for it shall be given you in that same hour what ye shall speak. …

Epistle LXXVI ↗

Gregory the Dialogist

But upon hearing so many terrors, the hearts of the weak could be disturbed, and therefore consolation is added when it is immediately subjoined: "Settle it therefore in your hearts not to premeditate how you will answer." As if He openly says to His weakening members: Do not be terrified, do not be afraid; you approach the contest, but I fight; you utter the words, but I am the one who speaks.

Forty Gospel Homilies, Homily 35 ↗

Bede

Therefore, set in your hearts not to meditate beforehand what you shall answer. For I will give you a mouth and wisdom, which all your adversaries shall not be able to resist or contradict. As if openly saying to his weakening members: Do not be terrified, do not fear. You go to battle, but I fight. You utter words, but I am the one who speaks.

On the Gospel of Luke ↗

Bonaventure

Third, as to the victory of those who are afflicted, he adds: Settle it therefore in your hearts not to premeditate how you shall respond, by your own inquiry, because, 2 Corinthians 3, "not that we are sufficient to think anything of ourselves as from ourselves, but our sufficiency is from God"; the Psalm: "The Lord knows the thoughts of men, that they are vain."

Commentary on Luke, Chapter 21 ↗