What does Psalm 44:18 mean?

Sin arises in the human heart (Jeremiah 17:9), but the psalmist claims here that the heart of the nation was free of guilt. The humiliating defeat of Israel (Psalm 44:9–16) doesn't seem to correspond to national sin or idolatry (Psalm 44:17). This makes the current situation impossible for the psalmist to understand, at least for now.

Israel was aware of the importance of loving God with the whole heart. Moses had commanded His people: "Hear, O Israel: The LORD our God, the LORD is one. You shall love the LORD your God with all your heart and with all your soul and with all your might" (Deuteronomy 6:4). The psalmist also claims that Israel had not gotten out of step with the Lord. This verse includes an assertion of loyalty to God. According to the psalmist, the nation had done what 85-year-old Caleb had done. In his request for Hebron, Caleb quoted what Moses had said about him: "Surely the land on which your foot has trodden shall be an inheritance for you and your children forever, because you have wholly followed the LORD my God" (Joshua 14:9).