Jamieson, Fausset and Brown on Psalms 118:1-4

PSALM 118

Psa 118:1-29. After invoking others to unite in praise, the writer celebrates God's protecting and delivering care towards him, and then represents himself and the people of God as entering the sanctuary and uniting in solemn praise, with prayer for a continued blessing. Whether composed by David on his accession to power, or by some later writer in memory of the restoration from Babylon, its tone is joyful and trusting, and, in describing the fortune and destiny of the Jewish Church and its visible head, it is typically prophetical of the Christian Church and her greater and invisible Head.

1-4. The trine repetitions are emphatic (compare Psa 118:10-12, 15, 16; 115:12, 13).

Let ... say--Oh! that Israel may say.

now--as in Psa 115:2; so in Psa 118:3, 4. After "now say" supply "give thanks."

that his mercy--or, "for His mercy."