A.T. Robertson Commentary Acts 23:11

"And the night following the Lord stood by him, and said, Be of good cheer: for as thou hast testified concerning me at Jerusalem, so must thou bear witness also at Rome." — Acts 23:11 (ASV)

The night following (τη επιουση νυκτ). Locative case, on the next (following) night.

The Lord (ο κυριος). Jesus. Paul never needed Jesus more than now. On a previous occasion the whole church prayed for Peter's release (12:5), but Paul clearly had no such grip on the church as that, though he had been kindly welcomed (21:18). In every crisis Jesus appears to him (cf. Acts 18:9). It looked dark for Paul till Jesus spoke. Once before in Jerusalem Jesus spoke words of cheer (22:18). Then he was told to leave Jerusalem. Now he is to have "cheer" or "courage" (θαρσε). Jesus used this very word to others (Matthew 9:2,22). It is a brave word.

Thou hast testified (διεμαρτυρω). First aorist middle indicative second person singular of διαμαρτυρομα, strong word .

Must thou (σε δε). That is the needed word and on this Paul leans. His hopes (19:21) of going to Rome will not be in vain. He can bide Christ's time now. And Jesus has approved his witness in Jerusalem.