John Calvin Commentary Romans 2:20

"a corrector of the foolish, a teacher of babes, having in the law the form of knowledge and of the truth;" — Romans 2:20 (ASV)

I take what follows, having the form of knowledge, as a reason for the preceding. It may be explained in this way: “because you have the form of knowledge.” For they professed to be the teachers of others, because they seemed to hold within themselves all the secrets of the law.

The word form stands for model (exemplar — pattern), for Paul adopted μόρφωσιν and not τύπον. Yet, I think he intended to point out the conspicuous pomp of their teaching and what is commonly called display. And it certainly appears that they were destitute of that knowledge which they pretended. But Paul, by indirectly ridiculing the perverted use of the law, intimates, on the other hand, that right knowledge must be sought from the law, in order that the truth may have a solid basis.