For we cannot but speak the things which we have seen and heard.

People's New Testament

4:20 We cannot but speak. Are under a moral and spiritual obligation to speak. They had a Divine message, and must tell it.

Jamieson-Fausset-Brown Bible Commentary

20. For we cannot but speak the things which we have seen and heard-There is here a wonderful union of sober, respectful appeal to the better reason of their judges, and calm, deep determination to abide the consequences of a constrained testimony, which betokens a power above their own resting upon them, according to promise.

Matthew Henry's Concise Commentary

4:15-22 All the care of the rulers is, that the doctrine of Christ spread not among the people, yet they cannot say it is false or dangerous, or of any ill tendency; and they are ashamed to own the true reason; that it testifies against their hypocrisy, wickedness, and tyranny. Those who know how to put a just value upon Christ's promises, know how to put just contempt upon the world's threatenings. The apostles look with concern on perishing souls, and know they cannot escape eternal ruin but by Jesus Christ, therefore they are faithful in warning, and showing the right way. None will enjoy peace of mind, nor act uprightly, till they have learned to guide their conduct by the fixed standard of truth, and not by the shifting opinions and fancies of men. Especially beware of a vain attempt to serve two masters, God and the world; the end will be, you can serve neither fully.