From https://www.gotquestions.org/apostasy.html:
Question: "What is apostasy and how can I recognize it?"
Answer: Apostasy, from the Greek word apostasia, means “a defiance of an established system or authority; a rebellion; an abandonment or breach of faith.” In the first-century world, apostasy was a technical term for political revolt or defection. And just like in the first century, apostasy threatens the Body of Christ today.
The Bible warns about people like Arius (c. A.D. 250 - 336), a Christian priest from Alexandria, Egypt, who was trained at Antioch in the early fourth century. About A.D. 318, Arius accused Bishop Alexander of Alexandria of subscribing to Sabellianism, a false teaching which asserted that the Father, Son, and Holy Spirit were merely roles or modes assumed by God at various times. Arius was determined to emphasize the oneness of God; however, he went too far in his teaching of God’s nature. Arius denied the Trinity and introduced what appeared on the surface to be an inconsequential difference between the Father and Son.
Arius argued that Jesus was not homoousios (of the same essence) as the Father, but was rather homoiousios (of similar essence). Only one Greek letter – the iota (i) – separated the two. Arius described his position in this manner: “The Father existed before the Son. There was a time when the Son did not exist. Therefore, the Son was created by the Father. Therefore, although the Son was the highest of all creatures, he was not of the essence of God.”
Arius was very clever and did his best to get the people on his side, even going so far as to compose little songs that taught his theology, which he tried to teach to everyone who would listen. His winsome nature and revered position as a preacher and one who lived in denial of himself contributed also to his cause.
With respect to apostasy, it is critical that all Christians understand two important things: (1) how to recognize apostasy and apostate teachers, and (2) why apostate teaching is so deadly.
Question: "What is apostasy and how can I recognize it?"
Answer: Apostasy, from the Greek word apostasia, means “a defiance of an established system or authority; a rebellion; an abandonment or breach of faith.” In the first-century world, apostasy was a technical term for political revolt or defection. And just like in the first century, apostasy threatens the Body of Christ today.
The Bible warns about people like Arius (c. A.D. 250 - 336), a Christian priest from Alexandria, Egypt, who was trained at Antioch in the early fourth century. About A.D. 318, Arius accused Bishop Alexander of Alexandria of subscribing to Sabellianism, a false teaching which asserted that the Father, Son, and Holy Spirit were merely roles or modes assumed by God at various times. Arius was determined to emphasize the oneness of God; however, he went too far in his teaching of God’s nature. Arius denied the Trinity and introduced what appeared on the surface to be an inconsequential difference between the Father and Son.
Arius argued that Jesus was not homoousios (of the same essence) as the Father, but was rather homoiousios (of similar essence). Only one Greek letter – the iota (i) – separated the two. Arius described his position in this manner: “The Father existed before the Son. There was a time when the Son did not exist. Therefore, the Son was created by the Father. Therefore, although the Son was the highest of all creatures, he was not of the essence of God.”
Arius was very clever and did his best to get the people on his side, even going so far as to compose little songs that taught his theology, which he tried to teach to everyone who would listen. His winsome nature and revered position as a preacher and one who lived in denial of himself contributed also to his cause.
With respect to apostasy, it is critical that all Christians understand two important things: (1) how to recognize apostasy and apostate teachers, and (2) why apostate teaching is so deadly.