Well according to Scripture, they were believers: But there rose up certain of the sect of the Pharisees which believed, saying, That it was needful to circumcise them, and to command them to keep the law of Moses. (Acts 15:5)
And Paul calls them "false BRETHREN" (Gal 2:4) which confirms that they were indeed believers, but holding to a false concept of Christianity. There are many such Christians today.
"False brethren" doesn't imply that they are believers in Gal 2:4.
If I call you a pseudo-Christian does that mean you are still a Christian? The prefix "pseudo" is commonly used, even in English, to indicate falseness.
The word in Greek for Gal 2:4 is
pseudadelphos. Pseudo means false, and delphos means brothers. So, they were not true brothers.
This means "false brothers"..those who are not true brothers.
However, in Acts 15, those individuals were likely believers who were simply mistaken and needed to learn that Gentiles were not required to live like Jews. This was an early stage of development of the Church.
So, my position would be that there are different grades of Judaizers..some are innocents who are simply misinformed (like Acts 15), some are false brothers (like the Judaizer teachers of Galatians and other areas mentioned throughout the epistles), and then there are those who are deceived by the false teachers.