Interestingly, in the book of John, John the Baptist is always referred to as John, never John the Baptist. However, in the synoptic gospels, John is often referred to as John the Baptist. If John the Apostle wrote the book of John, wouldn’t he more likely qualify with John the Baptist to distinguish him from himself?
https://debatingchristianity.com/forum/viewtopic.php?p=958315#p958315