Commentaries:
Verse 5 makes it clear to whom this psalm is addressed: "My saints . . . those who have made a covenant with Me by sacrifice." This warning is aimed at the church in general but specifically at those who fellowship with the church but are "wicked" by God's own judgment! Their wickedness identifies them as having departed from the way of God, even though they still give the outward appearance that they are "saints" by virtue of attending services. They are living in hypocrisy.
Human nature deceives us into thinking that God's patience with us—which gives us time to repent and change (Ecclesiastes 8:11; Romans 2:4)—is tacit approval of our conduct. Not so! He is testing us to see how serious our devotion and loyalty to Him and His purpose are. In reality, these wicked "saints" are not like Him, but human nature deceives them into ignoring this fact. They, like those of Matthew 7:23, will receive a devastating surprise in the judgment. They were warned! They may have even initially liked what they heard, but they were not motivated enough to depart from sin and correct their relationship with and witness of Him.
John W. Ritenbaugh
The Elements of Motivation (Part Two): Vision
The Message, a paraphrase, renders verse 21 as: “I kept a quiet patience while you did these things; you thought I went along with your game. I'm calling you on the carpet, now, laying your wickedness out in plain sight.” In other words, because the hammer had not yet fallen, the Israelites of old assumed that God did not care about what they were doing—but, as God declares, “. . . your sin will find you out” (Numbers 32:23).
Solomon speaks of this same tendency in Ecclesiastes 8:11. He observes that, because a delay in justice occurs, people conclude that crime and sin are not so terrible. This pause between cause and effect encourages them to continue in their evil.
God's longsuffering is thus double-edged. For those who are serious about following God, His longsuffering gives them time to repent and change. But for those who are ambivalent about what God thinks, His delay of justice reinforces their wrong beliefs and behavior. In this vein, the Israelites mistook God's silence for His approval, and as a result, the Day they thought would bring them glory instead delivered shame and defeat.
This pattern is evident in Israel's history, and we can also see it happening in the Israelitish nations of today.
David C. Grabbe
Do You Desire the Day of the Lord?