John Calvin Commentary Psalms 100:4

"Enter into his gates with thanksgiving, And into his courts with praise: Give thanks unto him, and bless his name." — Psalms 100:4 (ASV)

Enter his gates. The conclusion of the psalm is almost the same as its beginning, except that he adopts a way of speaking related to the worship of God that was practiced under the Law. In this, however, he merely reminds us that believers, in giving thanks to God, do not rightly discharge their duty unless they also continue in the practice of a steadfast profession of piety.

Meanwhile, in speaking of the temple, he indicates that God can only be worshipped in strict accordance with the way prescribed in His law. Furthermore, he adds that God’s mercy endureth for ever, and that his truth also is everlasting, to show us that we will never lack a constant reason for praising Him.

If, then, God never ceases to deal with us in this way, it would demonstrate the worst ingratitude on our part if we grew tired of offering Him the tribute of praise to which He is entitled.

We have elsewhere noted the reason why truth is connected with mercy. For we are so foolish that we scarcely feel God's mercy even while He openly manifests it, not even in its most palpable displays, until He opens His holy lips to declare His paternal regard for us.