A lot of biblical scholars think Ephesians wasn’t written by Paul, but by a student of his some time after his death.
F. F. Bruce addressed this issue in his commentary on Ephesians (published in 1961).
Bruce thought those scholars were giving Paul’s students too much credit. If one of Paul’s students wrote Ephesians, Bruce argued, then that student had advanced to the point where he could have taught Paul a thing or two.
Bruce concluded that Paul must have been the author. This was more likely, he thought, than that somebody had out-Pauled Paul.
In making this point, Bruce helps us to appreciate the brilliance of the letter:
If the Epistle to the Ephesians was not written directly by Paul, but by one of his disciples in the apostle’s name, then its author was the greatest Paulinist of all time—a disciple who assimiliated his master’s thought more thoroughly than anyone else ever did. The man who could write Ephesians must have been the apostle’s equal, if not his superior, in mental stature and spiritual insight. For Ephesians is a distinctive work with its own unity of theme. If we study it word by word and phrase by phrase, it may look like a compilation from the other Pauline epistles; but when we stand back and view it as a whole, it has an individuality and a message of its own. It was no mean judge of literary excellence, Samuel Taylor Coleridge, who described Ephesians as “the divinest composition of man.” Not only is it “the quintessence of Paulinism,” it carries Paul’s teaching forward to a more advanced stage of revelation and application than that represented by the earlier epistles. The author, if he was not Paul himself, has carried the apostle’s thinking to its logical conclusion, beyond the point where the apostle stopped, and has placed the coping-stone on the massive structure of Paul’s teaching. Of such a second Paul early Christian history has no knowledge.