If you google Paul false apostle you'll get hundreds of thousands of results. Of course, not all results contain opinions against Paul, but I estimate that at least half of them state that he departed from the true gospel of the Lord Jesus Christ.
There are also those who don't go to the extreme of calling Paul a false apostle, but regard him as "not completely reliable".
Is there any reason for us to think that Paul's authority is questionable?
Unfortunately, I have run across these people at different times. And for those who would consider Paul a false apostle and a liar, I present them with the following and that because they are guilty of the same:
"So then, dear friends, since you are looking forward to this, make every effort to be found spotless, blameless and at peace with him. Bear in mind that our Lord’s patience means salvation, just as our dear brother Paul also wrote you
with the wisdom that God gave him. He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand,
which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction."
So according to Peter, Paul's writings are wisdom from God and are to be considered to be scripture. Paul was caught up to the third heaven and continued to receive visions and revelations directly from the Lord. He healed, drove out demons and raised the dead and many other amazing works. Needless to say, God is not going to perform those things with someone who is a liar and a false apostle.
If we were to consider Paul as a false apostle and a liar and throw out his letters as not being God-breathed, then we must also throw out the writings of Peter, John and James who gave Paul the right hand of fellowship. And we must also throw out the book of Luke and Acts, for Luke who traveled with Paul supported him.
As the scripture states, those who hold this view of Paul distort the scriptures and are ignorant and unstable and they do so to their own destruction.