The back story:
There’s a lot going on in this passage, and little of it is actually about Paul!
The Pharisees and Sadducees are political and religious rivals locked in a perpetual power struggle for control of what is orthodox.
The Sadducees are the traditionalists, the elites, the ruling class. Historically they control the Sanhedrin and the temple. Politically, (for expedience sake), they are often in collusion with Rome. Religiously, they reject newer traditions (the Prophets) and therefore they are not only against the idea of angels, the spirit, and resurrection – but what they regard as Scripture does not contain the traditions of restorative justice for the oppressed.
On this analysis it would seem the Sadducees (and not so much the Pharisees) are diametrically opposed to Jesus, the Gospel, and the ministry of Paul.
Paul will of course know all of this. He is on trial at the hands of vested interests and powerful people.
So Paul does what Paul, and the Gospel he proclaims, does best – holds a mirror to those in power, and declares them a ‘whitewashed wall’, implying to the high priest that he does not follow his own laws and hides behind a thin veil of respectability.
At such a cutting remark I doubt it is simply Paul’s insolence which prompts the high priest to lash out. Paul has spoken the truth to power, which always provokes a response, usually a violent one.
Paul speaks this truth in the name of Jesus Christ, who could not be silenced, and if Paul is correct about the Resurrection, this truth could not be silenced even by death! If Jesus is raised from the dead the Sadducees have a lot to lose, indeed for Paul, they have already lost. Resurrection is the beginning of the end of the status quo, God is setting the world to rights, it has already begun.
So what began in Jesus, (or as Paul uses elsewhere in his defence, began with the Prophets) is proclaimed in word and deed by Paul, has been passed on to us. Not only to worship the God of Jesus and Paul in song, prayer, and preaching. Not simply to build a Church with open doors, hearts, and minds. But to hold a mirror to those in power, and proclaim justice not as punishment of transgressors, but as an equal sharing of the good things of the earth.
Not easy, so ‘keep up your courage’.