Throughout the Book of Psalms, we find a movement from despair toward praise and celebration.  Psalm 100 is one such high point.  It calls the whole world to praise the covenant God on two occasions (100:1-2 & 100:4).  Then, it provides the reason for this praise (100:3 & 100:5); God has brought redemption, saving and re-creating a People for Himself.  From this, we learn that God's Grace should promote our Gratitude.  Further, true Christian joy and praise comes from a consideration of His mighty works of salvation.  But, interestingly, the Old Testament people never experienced a time when nations could enter God's courts - Gentiles were "unclean."  Through this Psalm, the Old Testament believer looked forward to the time when God would save a multiethnic People through redemptive-judgment, bringing them blessing by crushing their enemies.  This judgment & salvation is the very thing sung about in the preceding psalms (Ps 93 & Pss 96-99).  And, throughout the Psalter, the Davidic King is never wholly absent - God's redemptive-judgment would, in fact, come through his hands (Ps 101).