How do we know that Diotrephes was not a true believer?

The character of Diotrephes illustrates a person who fundamentally opposes the gospel's principles. In 3 John 9-10, he is notably described as someone who does not receive those who bring the Word and actively opposes them, casting out believers instead. Such behavior indicates a lack of regeneration, as true believers, filled with the grace of God, would exhibit humility and support for the preaching of Christ rather than seeking personal preeminence. This clear contrast serves to identify him as not merely misguided but as someone who has not seen God, aligning with the broader teachings of Scripture regarding the evidence of one's faith.