What does Psalm 50:18 mean?

In this verse God, the ultimate Judge (Psalm 50:1–6) levels two charges against the defendants, the wicked Israelites (Psalm 50:7). These are specific accusations extending from the basic charge of hypocrisy (Psalm 50:16). Israel is offering sacrifices to the Lord, but He rejects them (Psalm 50:8–9). Their disobedience is not accidental; the nation in Asaph's time (Psalm 50:1) was deliberately and arrogantly throwing God's will to the side (Psalm 50:17; 73:2–3).

The first two examples given involve direct violations of God's basic rules for Israel: the Ten Commandments. First is theft (Exodus 20:15). Rather than challenging those who stole—directly or by dishonest means—they tolerate such evils (Romans 1:32). In the context of worship, this is especially offensive. The Lord asks in Jeremiah 7:11, "Has this house, which is called by my name, become a den of robbers in your eyes?" After arriving in Jerusalem to present Himself as Israel's Messiah, Jesus entered the temple and expelled the moneychangers. In that incident, He used similar language (Matthew 21:13).

The second charge given here is essentially identical but applies to the sin of adultery (Exodus 20:14). Sexual immorality and blatant thievery are not compatible with a worshipful attitude towards the Lord (Psalm 50:14).