Of course, he didn't know he was the Anti-Christ when he was born. Quite the contrary, he didn't know much of anything. But while he was yet very young, he made a very life-changing discovery.
"My fist is yummy!"
He spent weeks thereafter sucking on his fist, exploring the vast new realm of experience that had been opened up before him. Of course, he still did all the usual babyish things, such as eating, crying, and messing himself. But he considered them merely rote chores. They were tasks he felt compelled to do in order to free his mind for the great intellectual endeavor which lay ahead.
He felt there was no higher calling for a newborn in this life than to explore this strange phenomena of "yumminess of fist." He somewhat naively believed that he would become the world's foremost philosopher on the subject. Yes, it would be an arduous struggle, and he would be required to make many great sacrifices of time and effort in order to achieve his goals. But such is the path of a true intellectual.
Eventually, he was forced by circumstance and experience to leave this somewhat limited field of inquiry. He would have been quite hopeful to know that, in the waning years of their lives, many of the greatest minds of every age would return to the problem of "Why does my fist taste so yummy?" and spend the remainder of their earthly sojurn in happy contemplation of this very question.
The Anti-Christ moved on to more challenging questions, such as, "Who'se got my nose?" and "Why is mommy checking my diaper? Doesn't she know this is my 'hungry' cry?" Though neither of these questions were ever adequately answered, they led to other realms of inquiry, which reaped their own rewards.
How he became the Anti-Christ is anyone's guess. It is usually agreed, however, that it was probably just his bad luck. After all, Anti-Christs generally are frowned upon by The Big Guy Who Runs Things In Spite of What That Ninny Hawking Guy Says (BGWRTISWTNHGS), or "God," if you want to reduce His Holy Omniscient Character to a single syllable. It is also generally agreed that the Anti-Christ was going to burn in Hell for a few dozen eternities, discussing philosophy with Socrates while being impaled upon the Big and Pointy Stick of His Wrath. It is furthermore agreed that God, being omniscient, knew of the Anti-Christ's fate, and being omnipotent, could have made things turn out differently. Ironically, it is also generally agreed that the Anti-Christ brought this fate upon himself. This was a rather confusing set of agreements, which the Anti-Christ meant to ask Socrates about when he got a chance.