2 Timothy 3:16 All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness
You attend church one Sunday, and the pastor stands up and says “disregard what I taught on how one must be saved, this is now what I believe”. Or a prominent pastor on the radio you have listened to for years suddenly says “hey, I no longer believe Scripture is the inerrant word of God”. Or a commission is convened and they declare to your particular church “we now believe same-sex marriage and abortion are no longer sinful”.
Now if you were dependent on that pastor as opposed to Scripture, you would fall into their error of apostasy. Roman Catholics and Mormons have a huge glaring problem, because the deemed their teaching authority as “infallible”, so whatever the case might be, they must always follow their teaching as opposed to what Scripture teaches.
This is WHY by Scripture alone must be your practice. Man is corrupt, they are all fallible including myself. It must always be set ABOVE man and their teaching. So when and if a pastor goes off the rails, you can discern the apostasy trap immediately and leave that pastor and/or church. If you see the church as some sort of social club, where whatever they teach is not going to affect you, you are ripe for apostasy.
I said this before but I would venture to guess that 95 percent of Catholics sitting their pews would not care nor know the difference if their priest started teaching open Satanism. Because Scripture is NOT their authority and their church is set above it, they are literally ripe for any false teaching. Every cult will set man above Scripture.
Jesus and the Apostles constantly warned that the day will come when some with show themselves as apostates. Are you prepared? The only way to be prepared is by holding to Scripture alone, so when that day comes and the teacher says “I changed my mind, God is not sovereign” or “don’t trust the Bible, it has errors”, you will be ready not to affirm this lunacy, but to refute the lunacy, to challenge the lunacy.
Trust the Holy Spirit alone to guide you to the truth of God! Set His word ABOVE any man’s teaching which always needs to test out through that word. This is why by Scripture alone is so utterly important!