Philippians 4:4 - Ellicott's Commentary On The Whole Bible

Rejoice in the Lord... and again I say, Rejoice. — The original word is the word always used in classical Greek (see the corresponding word in Latin) for “farewell” (i.e., “Joy be with you!”), and this verse is obviously a resumption of Philippians 3:1, after the digression of warning. But the emphasis laid on it here, coupled with the constant references to joy in the Epistle, show that St. Paul designed to call attention to its strict meaning, and to enforce, again and again, the Christian duty of joy. It is, of course, a “joy in the Lord:” for only in the Lord is joy possible to any thoughtful mind or feeling heart in such a world as this.