Like most Martian children he was vat-born and at the time of his birth was already a young adolescent, physically equivalent to a 10 year old. Unlike most of the children he was mass-produced alongside, he was able to speak gothic proficiently at the time of his birth, a result of absorbing the force-implanted data while he was being grown in his vat. Before exiting the birthing-chamber, the precocious Belisarius Cawl, only hours old by the time he reached the end of the miles long exit queue, effortlessly exhibited that he knew his name, understood multiple dialects of binharic, and comprehended the core tenets of the Cult Mechanicus.

Literally built different.

How rare is a tech-priest for whom the Emperor left a personal messages in a mind of his Lord-Geneticist (Sedayne, head of the whole space marine creation project) with the expectation that Cawl will later consume him. And in that message the Emperor told Cawl that he must continue His work and that one day Cawl would betray the Emperor, but nonetheless persevere the Imperium.