Rejoicing refers to moderation of spirit when the mind keeps itself in calmness under adversity and does not give indulgence to grief. Constant praying is the way of 'rejoicing perpetually', for by this means we ask from God alleviation in connection with all our distresses.
John CalvinFor though we very truly hear that the kingdom of God will be filled with splendor, joy, happiness and glory, yet when these things are spoken of, they remain utterly remote from our perception, and as it were, wrapped in obscurities, until that day.
John Calvin