He turned the sea into dry land: they went through the flood on foot: there did we rejoice in him.
John Calvin on Psalm 66:6:
The Psalmist passes next from the more general point of his providence towards mankind at large, to his special care over his own Church, adverting to what he had done for the redemption of his chosen people. What he states here must be considered as only one illustration of many which he might have touched upon, and as intended to remind God’s people of the infinite variety of benefits with which their first and great deliverance had been followed up and confirmed. This appears obvious from what he adds, there we rejoiced in him. It is impossible that the joy of that deliverance could have extended to him or any of the descendants of the ancient Israelites, unless it had partaken the nature of a pledge and illustration of the love of God to the Church generally. Upon that event he showed himself to be the everlasting Savior of his people; so that it proved a common source of joy to all the righteous.
Source: http://www.puritanboard.com/showthread.php/88837-A-common-source-of-joy, Comment #1