od is who He is. He was God before He made humanity. He will be the same 10,000,000 years after the earth has passed away (Malachi 3:6). He is mighty (Psalm 50:1). He is not like us (Psalm 50:21). And, importantly, He doesn't answer to anyone (Psalm 50:6).
Perhaps the most amazing thing about God's greatness is that He voluntarily condescends to consider us. After all, He doesn't exactly need us, nor does He need our resources (Psalm 50:12). Now, in this psalm we find that God handles the wicked very differently than He deals with His covenant people. For those of us who are safe in Him, God acts as our defender and gathers us unto Himself (Psalm 50:4-5). He comes to us... He speaks to us, and against the unbelieving and unfaithful among us.
Notice that His rebuke is for some who are among us, not (in this case) against outsiders. He calls out insiders who talk the talk, but who won't walk the walk (Psalm 50:16-17). God asks, "Why do you simultaneously declare My doctrine, while you are personally ignoring it?" He even names a few of the vices of these pretenders (Psalm 50:18-20)... they join in on evil when they discover it, they are promiscuous, they are deceptive with their tongues and they are disloyal talebearers. And, lest we imagine that any among us can get away with such duplicity, God promises to reprove all who are guilty of these infractions (Psalm 50:21) and to judge the unrepentant (Psalm 50:22).