Galatians 5:4-5
You have been severed from Christ, you who are seeking to be
justified by law; you have fallen from grace.
Christians may fall from grace, but it is not accomplished by sinning.
They will have fallen from grace when they, having been delivered from the Law with its merit system, and then turn back to the merit system. (That is falling from Grace -- falling back into the Law).
"No person who has gained the understanding of what it means to be perfected in Christ, beyond the need of any human works to complete that perfection, should ever turn back to the Law or personal merit for salvation. People who trust Christ as Savior are perfected in Him whether they realize it or not. It is those that do not realize it, who may be influenced by legalists to turn back to the merit system from which they have been delivered, that fall into this deception."
(Systematic Theology Chafer, vol 3 pg. 31
There are only two systems: law and grace. When a person
falls from grace, there is only one place to fall - that is back
into the Law. ( Personal Works Salvation)
Unsaved people may learn of the grace of God; but when
they turn from that knowledge and go back into the
legalistic merit system, they become severed from Christ
with no way to be indwelled with the Holy Spirit and have
their sins forgiven (Be Saved). Hebrews 10:26
Galatians 2:21
I do not nullify the grace of God; for if righteousness comes
through the Law, then Christ died needlessly .
The sin of rejecting the Lord Jesus Christ and His forgiveness
for sins is the sin, that if continued in, cannot be forgiven.
This is simply because there is no other means to be forgiven.
Hebrews 10:26
For if we go on sinning willfully after receiving the knowledge of
the truth, there no longer remains a sacrifice for sins,
As to those that have been "Born Again" indwelt with the Holy Spirit:
Hebrews 10:39
But we are not of those who shrink back to destruction, but of those who have faith to the preserving of the soul.