I'll try to say this as simple and basic as possible. GOD is referring to those who SAY they believe but do not live the true beliver's Christian lifestyle. In other words, they talk it but do not walk it. Just because someone says they are Christian but they continue on disregaridng GOD's commandments and continue in disobedience, they are not true believers. They will depart from GOD (eternal damnation). Naturally, GOD is referring to those who are purposely evil as well such as false prophets.
Please understand I am not referring to true believers who sin (fall) and then seek forgivenes, continue on *wanting* to grow with the Lord. These are two different kinds of people and GOD surely knows every man's heart.
Matthew 7:23
And then I will declare to them, ‘I never knew you;
depart from Me, you who practice lawlessness!’
Matthew Henry's Commentary:
Nothing so much prevents men from entering the strait gate, and becoming true followers of Christ, as the carnal, soothing, flattering doctrines of those who oppose the truth. They may be known by the drift and effects of their doctrines. Some part of their temper and conduct is contrary to the mind of Christ. Those opinions come not from God that lead to sin. (Mt 7:21-29)