https://salvationcall.com/apostates/
Apostates are those who have committed apostasy. Apostates used to be followers of Jesus, but they ceased to be followers along the line. Apostates are devout Christians who have abandoned Christianity and are no more Christians.
Taking this Scripture into consideration, if at any point in time, a Christian turns away from the truth or true doctrines and embraces false doctrines, he ceases to be a true worshipper and child of God; rather, he becomes an apostate. Those Christians who become apostate by embracing and practicing false doctrines do not become unbelievers as we understand the term unbelievers, for they still maintain that they are Christians, and that they are part of the Christendom, and that they believe in Jesus, and that they teach from the Bible; at least they may have few opinions about Jesus or few Biblical doctrines that are true.
However, they have quite a number of false doctrines that contradict the core beliefs of the Christian faith. Christians who apostatize by embracing false doctrines form a category of apostates who are well known as heretics. Heretics are a category of apostates who either believe and practice entirely false doctrines or they believe and practice a synthesis of true and false doctrines. Heretics teach their false doctrines to other Christians and non-Christians alike to convert them to become heretics.
One notable heretic in recent history is Charles Taze Russell, founder of Jehovah’s Witnesses. Russel managed to teach his false doctrines to others and won as many as possible to start the denomination which is now called Jehovah’s Witnesses. Though Jehovah’s Witnesses has many unbiblical doctrines, they still count themselves as Christians. Other examples are Joseph Smith, founder of Latter-Day Saints.