Bartholomew was Nathaniel,who was mentioned in the John 1. He was introduced to Christ through Philip.

Philip and Bartholomew are always mentioned together.

45 Philip found Nathanael and told him, "We have found the one Moses wrote about in the Law, and about whom the prophets also wrote--Jesus of Nazareth, the son of Joseph."

46 "Nazareth! Can anything good come from there?" Nathanael asked. "Come and see," said Philip.