Ah you foolish Galatians, who has bewitched you?
One can start out well, stray off the narrow path to the left or right as a backslider or into legalism, and find your way back to the path. It is not salvation that I had in mind (which was settled before the foundation of the Earth) but the “walk” (sanctification).
What you are describing is that of heresy, not apostasy.
The distinction is in terms of renunciation. One may certainly drift, as some say, stray, however the apostate doesn't drift or stray, they actually renounce or state in effect that they no longer hold to certain core doctrines and foundational statements of Scriptures.
One may be involved in heresy before continuing to apostate by actually declaring that which is opposite to Biblical truth.
Back decades ago, a man named Peale was an influential fellow. A denier of many of the fundamental foundational statements of Scriptures. A "bishop" who stated, "It is not necessary to be born again." He was an apostate. Though he carried great credentials and influence, he was a denier of the foundational truths of Scriptures.
There are those who would lump Hyles, and Ruckman into the camp of apostasy, I do not extend to them that dishonor, but I do hold both as heretical in some of their thinking and teaching.
Now, I give you these three names not to shame them, but to give you as loose examples to further your own investigation into the difference and why I can hold Hebrews 6 as I do.
It is important that you also keep in play that thinking given by the Apostle Paul, that one of the indications of the last days is the acceptableness of the apostasy, and people desiring their ears be pleased rather than their hearts tuned to the Holy Spirit by being taught the Scriptures.
The apostate is as Judas the betrayer. He was well respected in the apostles, trusted as the keeper of the treasury, entrusted to walk with them, learn with them, do miracles with them, and yet he was never "of them."