What does Acts 23:8 mean?

Paul is using theological differences between Sadducees and Pharisees to make them forget he insulted their high priest. The Roman tribune brought Paul before the Sanhedrin, hoping the religious council could tell him why a mob of Jews dragged Paul from the temple and beat him (Acts 21:27–34). Paul has a different goal. He wants to tell the religious leaders about Jesus. He's barely begun when the high priest has Paul struck and he responds by calling the priest a whitewashed wall. When Paul finds out who had him struck, he obliquely calls the high priest corrupt…which he is (Acts 23:1–5).

Quickly, Paul realizes everything is going downhill. Yet he also realizes the consequences don't all need to land on him. He declares that he is a Pharisee and that he is on trial for affirming the resurrection of the dead. The Pharisees believe in the resurrection, but the Sadducees don't. They start fighting, and the tribune has no option but to withdraw with Paul in tow (Acts 23:6–7).

The belief that resurrection from the dead is part of the Messianic message came about after the exile to Babylon; there's almost no clear resurrection theology in the Old Testament, but that doesn't mean it isn't true. Jesus explained this to the Sadducees shortly before His crucifixion. God told Moses He "is" the God of Abraham, Isaac, and Jacob, not that He "was" their God. He cannot be the God of the dead (Mark 12:26–27).