In Galatians 6:12, 13 we find four marks of false teachers to be on the look out for.
The first is that they are out to impress, Paul suggests that part of the motive the false teachers have for insisting on circumcision is that they are out to impress people by means of the flesh. They know that for some circumcision will be difficult and so in a rather fleshly way they insist on it as a means of impressing people.
This is something to watch out for with false teachers. They insist on one fleshly requirement or another partly in order to impress people. They insist on a certain period of fasting or of vigil or wearing certain clothes or speaking in tongues or some other marker that is simply a fleshly act designed to impress people.
To take two examples. On the last day of July in Ireland people climb the mountain Croagh Patrick barefoot in an act of devotion, mass being celebrated at the summit. Or even more extreme, every Easter in the Philippines certain men will flog themselves with whips and even get themselves crucified (usually without actually dying). Such acts of the flesh are designed ultimately to impress people. What devotion! What commitment! Don't be fooled.