"At the heart of the marriage relationship is the idea of faithfulness to the vows of commitment.
Faithfulness is important both in marriage and in the people's relationship with God. Their unfaithfulness to God is considered spiritual adultery."
-quote from the Chronological Bible comments

Spiritual adultery happens when believers in Jesus break their vows of commitment and follow after other gods.

According to the law, the penalty for adultery is the death of both parties involved.
(Deut 22:22, Lev 20:10)

When the teachers of the law and the Pharisees brought to Jesus a woman caught in adultery, notice they didn’t bring in the man with her. (John 8:3-11) Could it be that some of those men who were standing around the woman were guilty themselves? Is that what Jesus wrote on the ground…their names or their sins?

All the woman's accusers left, one by one. Jesus told the woman that He didn't condemn her. But He also declared. "Go now and leave your life of sin."

The sin is condemned, but not the repentant sinner. Praise God for that!