Without controversy, the mystery of godliness is great: God was revealed in the flesh, Justified in the spirit, Seen by angels, Preached among the nations, Believed on in the world, And received up in glory.
The widely held opinion that this is from some earlier hymn in current use among the earliest Christians is totally without foundation.  The supposition is based upon the rather indefinite word which is here rendered "he," but which is also sometimes rendered "which" or "who."  Any way it is translated the meaning is exactly the same, namely, that of the KJV which rendered the passage "God was manifested in the flesh," which is exactly what the passage SAYS.  "He who" refers to "God" mentioned twice in the preceding verse, and "which" or "who" would also have the same reference to "God."  And, of course, it is fully consonant with everything else in the New Testament.  God was manifested in the flesh in the person of Christ. In support of the hymn theory, it is frequently pointed out that there are rhythm and balance, etc.; but many of Paul's statements have the same qualities, notably in the salutation of  Romans 1:1-7, which see in my Commentary on Romans.  This writer accepted the hymn theory regarding this passage in "The Mystery of Redemption," written several years ago, but more mature studies have raised serious misgivings about the various "hymns" said to have been quoted by Paul.  Anyway, even if it is a hymn, the odds are overwhelmingly in favor of the proposition that Paul himself was the author of it.