A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 100:1As closing this series (see on Psa 93:1), this Psalm is a general call on all the earth to render exalted praise to God, the creator, preserver, and benefactor of men. (Psa 100:1-5)
With thankful praise, unite service as the subjects of a king (Psa 2:11-12).
psa 100:3To the obligations of a creature and subject is added that of a beneficiary (Psa 95:7).
psa 100:4Join joyfully in His public worship. The terms are, of course, figurative (compare Psa 84:2; Psa 92:13; Isa 66:23).
Enter--or, "Come with solemnity" (Psa 95:6).
psa 100:5The reason: God's eternal mercy and truth (Psa 25:8; Psa 89:7).