The difficulty in attempting to explain the meaning of a passage of Scripture to one who has his mind made up already is a task greater than a human being possesses. Aren’t you glad that it’s the Holy Spirit Who illumines believers’ minds—and not you?
Of course, He does so—interestingly—not apart from but, through the Word itself.
It is, therefore, crucial when dealing with pig-headed Christians, who think they are accomplished exegetes, but can’t tell the difference between the meaning of a verse from a child’s jingle, to remember this and to do what you can do.
The first profitable thing to do is to refrain from argument, reason or trying to beat the truth into his head by pure repetition.
“I understand that—but what can I do?”
The thing to do—at all costs—is to get him to read (rather, study) the Bible.
“How will that help, if his mind is made up?”
Since, as Paul said, the truth is “spiritually discerned,” that is where your hope lies. When he gets serious about learning what God says from the Bible, you can expect things to happen. For truth to be spiritually discerned means, to have the Spirit working in him to enable him to understand His Word.
And that’s exactly what you want—isn’t it? Not what you think, but what the Spirit teaches from His Book.
So, don’t argue. Instead give him helps that will encourage him to study the Scriptures—concordances, Bible dictionaries, commentaries—whatever it takes to get him into serious study of the Bible.
If you succeed in persuading him do so, you’ve won twice over: in time he may soon come to see the truth about what you said, and—of greater importance—he will become a student the Word of God. Even if it takes time, humility, or even repentance for the former to occur, in the meantime you can rejoice in the latter.