Acts 4:20

For we cannot but speak (ou dunameqa gar hmei--mh lalein). Both negatives hold here, "For we (note emphatic hmei) are not able not to speak" (what we saw and heard). This is defiance of the civil and ecclesiastical authorities that was justified, for the temple authorities stepped in between the conscience and God. Peter and John were willing to pay the price of this defiance with their lives. This is the courage of martyrs through all the ages.