Rejoice in the Lord. It is an exhortation suited to the times; for, as the condition of the pious was exceedingly troublous, and dangers threatened them on every side, it was possible that they might give way, overcome by grief or impatience. Hence he enjoins it upon them, that, amidst circumstances of hostility and disturbance, they should nevertheless rejoice in the Lord, as assuredly these spiritual consolations, by means of which the Lord refreshes and gladdens us, ought then most of all to show their efficacy when the whole world tempts us to despair. Let us, however, in connection with the circumstances of the times, consider what efficacy there must have been in this word uttered by the mouth of Paul, who might have had special occasion of sorrow. For if they are appalled by persecutions, or imprisonments, or exile, or death, here is the Apostle setting himself forward, who, amidst imprisonments, in the very heat of persecution, and in fine, amidst apprehensions of death, is not merely himself joyful, but even stirs up others to joy. The sum, then, is this — that come what may, believers, having the Lord standing on their side, have amply sufficient ground of joy.
The repetition of the exhortation serves to give greater force to it: Let this be your strength and stability, to rejoice in the Lord, and that, too, not for a moment merely, but so that your joy in him may be perpetuated. For unquestionably it differs from the joy of the world in this respect — that we know from experience that the joy of the world is deceptive, frail, and fading, and Christ even pronouces it to be accursed (Luke 6:25). Hence, that only is a settled joy in God which is such as is never taken away from us.