Stephen never spoke against Moses, the Torah, or God. If he had, his enemies would not have had to pay false witnesses to claim he did. Acts 6:11-15
When Stephen defended himself before the Jewish court, he didn't answer their questions directly. He took the opportunity to preach the truth to all who would listen.
But they could not withstand the wisdom and the Spirit with which he was speaking.
Acts 6:10
The Holy Spirit is irrefutable. When the Spirit speaks through a man, he speaks only what is true and required. No man can possibly out maneuver or out talk God.
The real trick is determining what spirit is speaking when someone claims to be prophesying.