Ebionites were a Jewish-Christian group that believed that Jesus did not exist before his birth and that God adopted him at his baptism.
Like so many other heresies, this false belief removes Jesus from the Godhead and teaches the falsehood of Adoptionism.
So, who were the Ebionites?
The term itself means "the poor," and they were a religious group of Jewish-Christians who believed in following the Old Testament laws.
Reading the New Testament clarifies that we need Jesus because no one can keep the law. Paul states, "Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God." Romans 3:19
"For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh:" Romans 8:3
Other heresies taught by the Ebionites were the denial of Jesus being God in the flesh. The Bible is clear about Jesus being God in the flesh and states as much in many verses, including Matthew 16:15-18; John 14:9-10; John 1:1-14; Philippians 2:6-7; Colossians 2:9, and many more.
They also taught that Jesus was created and God adopted Him at the time of His baptism. This heresy is Adoptionism, which I address in another Heresy 101 article.
The Ebionites were vegetarians, rejected Paul's teachings, and accepted only the Gospel of Matthew.
Ironically, Matthew has many scriptures that refute and corrects most of the Ebinoites heretical beliefs.
Among other things, they denied the virgin birth (which Matthew 1 accounts for, thus refuting the Ebionite's heretical claims). Still, they did affirm the crucifixion and resurrection, both meaningless if Christ was not God in Flesh and of the virgin birth.
References
Geisler, N. L., & Nix, W. E. (1996, c1986). A general introduction to the Bible, Chicago: Moody Press, p. 302 citing The Oxford Dictionary of the Christian Church, pp. 438-39.