Psalms 50:18

When thou sawest a thief, then thou consentedst with him, and hast been partaker with adulterers.

To see a thief and agree with him, and to join with adulterers, is to become a guilty sharer and partner in their theft and adultery, for approval of wrongdoing counts as participation in it. That sharing in evil reaches to lustful conduct and to sanctioning evil and tempting others to sin, drawing them into it, so that approving it and leading others into it are themselves the condemned partaking. Such complicity stands in direct contradiction to claiming to belong to God, to be in covenant with Him, to serve Him, or to be holy, since conduct that consents to theft and joins adultery denies the profession of faith and covenant.

A holy appearance does not prove the heart is right, for deeds reveal character and expose hypocrisy, including casting aside God's words and living deceitfully while professing holiness. That corrupt conduct brings reproach upon God's cause and gives its opponents reason to criticize. In the setting of judgment and the call to repent, joining in or approving wrongdoing, including theft and adultery, is warned against as part of the character being recorded before God.

In the wider setting of Psalm 50, which shows the greatness of God, the verse warns those working on Satan's side what a fearful loss they are incurring. At the last great day those given to anarchy, war, and strife will see how many souls were lost because they failed to take their stand under the rule of Christ.