Philip found Nathanael, and said to him, "We have found him, of whom Moses in the law, and the prophets, wrote: Jesus of Nazareth, the son of Joseph."
If Nathaniel was not indeed an apostle, the same man as Bartholomew, how can it be explained that Christ appeared after his resurrection to a group of seven, and, of the five named, all were apostles except Nathaniel?  That such a list of named apostles included one who was not an apostle is extremely unlikely (John 21:2).