Who was the 12th Apostle to replace Judas, and why was Judas no longer considered an apostle?
The twelfth apostle to replace Judas was Matthias, as described in Acts 1.
Matthias is mentioned in scripture only in that chapter, but his low profile is not unusual. The lives of most of the twelve apostles are given little or no attention in scripture. Only a handful (e.g., Peter, John, James) are given an extended mention because of their leadership roles in the first century. Nevertheless, the Bible is clear on who were the first twelve apostles of the church, and though Judas was called as an apostle for a time, he was never intended to fulfill that role, as Peter says:
Scripture tells us that Judas was deceived by Satan (John 13:2), he was indwelled by Satan (John 6:71), He betrayed Christ (Matt 26:25), he willingly chose to give up his apostleship to serve the devil (Acts 1:25), he later felt remorse for his sin (Matt 27:3), and he died a dishonoring death in keeping with his wicked actions (Acts 1:18).