Primary function of brahmin

I am new to this forum. After reading few posts, I started wondering: What was the primary function of a brahmin in ancient land of ours - was it practice [ayurveda / siddha physicians, priesthood, etc] or was it spreading knowledge [gurus]? If that's so, how does it relate to today? What makes one a true brahmin?

As far as I understand, doing kriyas / sandhi is only one part of being brahmin. Please enlighten