That the "doxology" is a postscript added by the apostle after he had arrived at, and was residing in, Rome (61-63 A.D.: see Appendix 180), and was writing Ephesians, seems clear for the following reasons:—
First, there is no question as to the genuineness or authenticity of these verses.
The question raised by their appearance not only after the close of the Epistle itself, but also after the postscript of the amanuensis, Tertius, is connected with the "mystery" "kept in silence from age-times but now manifested by means of prophetic writings". To find the subject-matter of Ephesians introduced suddenly, in such a position, and in the diction of this doxology, has been a difficulty for ancient transcribers and modern commentators alike.
The original Manuscripts prove this by the position the doxology occupies in many of them.
In over 190 it stands after Romans 14:23.
In two or three it is wanting.
In some it appears in both places (that is to say, after Romans 14:23 and 16:24).
In some, where the doxology stands as in the Authorized Version the second benediction (verse 24) is omitted.
This difficulty is shared by modern commentators. Some suppose the doxology was "the effusion of the fervent mind of the apostle on taking a general view of the Epistle".
Others say—"it needs only to read the doxology to see that its main purpose is nothing lower than thanksgiving for the Universal Gospel as a whole, and that its weighty grandeur of tone belongs to the close not of a section, but of the whole Epistle."
But the suggestion that this "postscript" was added later by the apostle removes all the difficulties, and shows that the minds of the ancient copyists were needlessly disturbed. The truth of the "mystery" had been lost long before the date of our oldest Manuscripts. Hence the transcribers' excitement and perplexity. Had it been known, they would have at once understood that the doxology was subsequently added.¹ And the same remark applies to modern commentators.
Although Paul must have had the "secret" revealed to him beforehand, probably about 57 or 58 A.D., yet he was not permitted to publish the truths of the mystery in writing until after he was in Rome, and in prison. Consequently, when the Epistle was sent first to the Romans, it was closed by the second benediction (verse 24).
Although given to him before the expiry of the period of grace enjoyed by the Pentecostal church, he was not allowed to divulge it. So long as the offer of the Kingdom (see Appendixes 112, 113, 114) to earthly Israel was open, the "mystery" could not be made known.
But when the sentence of judicial blindness had been promulgated and the prophecy of Isaiah 6 fulfilled (Acts 28:26, 27), then the glorious truths for the later-born were allowed to be set forth by "prophetic writings", namely the prison epistles.
Therefore the apostle was guided by the Holy Spirit to add the postscript to Romans; thus completing in beautiful perfection the Divine arrangement of the Epistle (see Structure, page 1661) and striking the key-note in the doctrinal teaching which is taken up and developed at large in Ephesians.
¹ This suggestion was first made by Bishop Lightfoot in Biblical Essays, and adopted by others.