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?
Greetings Marcelo,
The answer to why there are those who proclaim that Paul is a false apostle and that his writings are not God-breathed, is just another deception from Satan. Regarding this, consider the following:
"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." 2 Pet.3:15
First of all, Peter refers to Paul's writings as wisdom from God and as being scripture. Notice that it states that there were people of that time who were distorting the scriptures that Paul had written, who are referred to as ignorant and unstable and that by doing so it is to their own destruction.
By discrediting Paul and all that he wrote, they are speaking against the Lord and His word, because it is from the Lord that Paul received his information from. Since it is said that by distorting Paul's writings they do so to their own destruction, then Satan knows that if he can get people to hold the same teaching about Paul and his writings, then they will do so their own destruction as well.
We know that Paul was caught up to the third heaven and that he received visions and revelations directly from the Lord. These people who are against Paul and his writings have been deceived by Satan and his false teachers and if they continue in that, it will be to their own destruction.