The false testimony against Stephen was that he was saying things about the temple and the Mosaic Law that the Jews regarded as untrue and unpatriotic (cf. Matthew 26:59-61). Stephen appeared to be challenging the authority of the Pharisees, the Mosaic Law, and a major teaching of the Sadducees, namely, the importance of the temple. He was evidently saying the same things Jesus had said (cf. Matthew 5:21-48; Matthew 12:6; Matthew 24:1-2; Mark 14:58; John 2:19-21). [source][source][source]
"Like the similar charge against Jesus ( Matthew 26:61; Mark 14:58; cf. John 2:19-22), its falseness lay not so much in its wholesale fabrication but in its subtile and deadly misrepresentation of what was intended. Undoubtedly Stephen spoke regarding a recasting of Jewish life in terms of the supremacy of Jesus the Messiah. Undoubtedly he expressed in his manner and message something of the subsidiary significance of the Jerusalem temple and the Mosaic law, as did Jesus before him (e.g, Mark 2:23-28; Mark 3:1-6; Mark 7:14-15; Mark 10:5-9). But that is not the same as advocating the destruction of the temple or the changing of the law-though on these matters we must allow Stephen to speak for himself in Acts 7." [1][source]
"For Luke , the Temple stands as a time-honored, traditional place for teaching and prayer in Israel, which serves God"s purpose but is not indispensable; the attitude with which worshippers use the temple makes all the difference." [1]6 [source]