I was ceremonially clean when they found me in the temple courts…
The Law of Moses and the traditions surrounding it required many types of ritual cleansings. Jesus clashed with the religious authorities about some of the traditions. Hand washing before meals, for example.
However, there were special requirements of cleanliness before one could enter the temple and the presence of God. Failure to do so was a form of desecration of the temple.
Desecration of a temple of any kind was a serious offense, not only to the adherents of the particular religion, but also to Roman authority.
Part of the success of Roman governance was a respect for different religions within the Empire. With Paul standing before the governor, a charge of temple desecration, if proven, will prove most damning to Paul.
This is the angle the prosecutor, Tertullus, presses against him. And the reason he gives to justify seizing Paul in the first place.
But Paul was ceremonially clean, according to the Jewish customs, when he entered the temple and he says so. And offers in his defense - They cannot prove to you the charges they are now making against me.
But Paul knows that ceremonial cleansing of the body does not make a person truly clean. Washing the outside cannot remove the guilt of sin on the inside. Only trust in Jesus and His sacrifice can make a person clean before God.
This is the message Paul has been preaching. And his success in turning people to faith in Jesus is really why he is in trouble here.
Paul deftly turns the trial away from his personal behavior to the real issue - It is concerning the resurrection of the dead that I am on trial before you today. Even on trial for his life, Paul stays on message!