"the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons,"
" Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God."
Yes, its always about belief or unbelief, aka having faith as opposed to not having faith.
Hebrews 3[
18] And to whom sware he that they should not
enter into his rest, but to them that believed not. Matthew 11:28-30 and John 3:5-7.
I do understand the perspective that you are attempting to deliver, but it is not applied to those who are "born again" of His Holy Spirit.
Mere "believers" are counted as brethren, because of the simple faith that they profess.
However, until they confess openly/publicly that Jesus is their Lord and Savior, and are baptized by water and of the Holy Spirit, their faith is only that of
a mere belief that God is, which is not evidence of their salvation through the Lord Jesus.
Within the churches, there are many who attend and do appear to believe in God, but they don't have faith to come to Him, in repentance through Jesus.
It is that level of faith that is subject to hardness of heart through sin, which causes a person to "fall away" from having faith in the Living God.
[
19] So we see that they
could not enter in because of unbelief.
Iows, they
could not enter into the KoG through the new spiritual birth by His Holy Spirit, aka being "born again". Hence the reasoning of the Holy Spirit through Paul: Romans 8:9 "...Now if any man
have not the Spirit of Christ, he is none of his.
We see this situation of people believing in Jesus, but as yet HAD NOT received the Gift of the Holy Spirit, which SEALS a believer unto the Day of redemption. Acts 19:1-6 Ephesians 4:30
[
1] And it came to pass, that, while Apollos was at Corinth, Paul having passed through the upper coasts came to Ephesus: and finding certain disciples,
[
2] He said unto them,
Have ye received the Holy Ghost since ye believed? And they said unto him, We have not so much as heard whether there be any Holy Ghost.
[
3] And he said unto them, Unto what then were ye baptized? And they said, Unto John's baptism.
[
4] Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.
[
5] When they heard this, they were baptized in the name of the Lord Jesus.
[
6] And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.