John Calvin Commentary Acts 15:5

"But there rose up certain of the sect of the Pharisees who believed, saying, It is needful to circumcise them, and to charge them to keep the law of Moses." — Acts 15:5 (ASV)

Certain of the sect of the Pharisees. It is for good reason that Luke makes clear what kind of men they were who sought to trouble or hinder Paul, even in Jerusalem as well. And it is thought that the evil originated from that source; and that Luke now more plainly expresses that disturbers now also emerged from that very same sect, from where the authors of that wicked dissension originated.

For though they had professed faith in Christ, yet remnants of their former nature remained. We know how proud the Pharisees were, how haughty, how arrogant they appeared; all of which they would have forsaken if they had truly put on Christ. Just as no Pharisaism remained in Paul, many of them had acquired a habit of stubbornness through long custom, which they could not easily shake off immediately.

Because hypocrisy especially reigned among them, they were excessively devoted to external rites, which are coverings for vices. They were likewise puffed up with pride, so that they tyrannically desired to make all other people subject to their decrees. It is well known how severely afflicted monks are with both these diseases. Consequently, none are more cruel than they in oppressing the Church, and none are more wicked or presumptuous in despising the Word of God. Moreover, we see many who came out of those dens, who have cast off their cowl, and yet they can never forget the dispositions they learned there.