'Unfaithful' in the Bible

Carmi’s son: Achar, who brought trouble on Israel when he was unfaithful by taking the things set apart for destruction.

But they were unfaithful to the God of their ancestors. They prostituted themselves with the gods of the nations God had destroyed before them.

So Saul died because he was unfaithful to the Lord and did not obey the Lord's instructions; he even tried to conjure up underworld spirits.