1 Jn 2:19 is a good place to start. . .then there's Mt 13:24-30, Lk 8:13, Jn 1:1, 1 Co 11:19.Be kind of hard for anyone to judge false believers unless you are God. Have you any verses that testifies to false believers?
Paul contrasts wicked and evil men without faith (2 Th 3:2), who are tares, to those of faith , who are wheat and whom the Lord strengthens and protects from the evil one (2 Th 3:3).And yet 2 Thessalonians 3:1-7,14 & 15 KJV testifies about those that have fallen away from the faith and are disorderly, that we are commended t withdraw from them with the Lord's help but not to treat them as the enemy, bit admonish them as brothers because even if they wind up left behind, the Good Shepherd, will get that lost sheep.
There are both wheat and tares in the kingdom of God (Mt 13:24-30), and they are allowed to grow together, to be separated and burned only at the end.
Such was the situation in Thessalonica.