It was very obvious in the early days of the church to identify when someone had fallen away from the faith.  They left the real church and went to follow false teachers who clearly were not sanctioned by the apostles.  In our day, they just hop around to different churches in order to find some safe ground so that their lives won’t be called into question.  But here is what is clear from 1 John 4:4-6—those who are from God and know the Spirit do not get led astray into false teaching.  Let me be clear here—God can and does save from false teaching, but when someone is around the true teaching and then drifts into false teaching, that is a sign of extreme danger and probably proves they are void of the Spirit.  That is the falling away described in Hebrews 6.
Look at v.5, where John states that the false teachers are from the world, which is why the world listens to them.  However, the Spirit keeps his children from listening to them (v.4), because he is greater than the spirit of disobedience at work in those false teachers.  It follows then that whoever refuses to listen to the apostles’ teaching is not born again from God and indwelt with the Spirit.  Yes, we are very patient when giving the truth to someone, just as God has been patient with our lack of discernment, but if someone remains hostile or even indifferent to the truth, then there is no reason to assume that person is in Christ.  If they are susceptible to the spirit of error, how can it be said that the Spirit of Truth resides in them?