Abraham knew God at three but the Torah doesn't really tell his story until he's 75, when God tells him to leave everything. That command to go, to leave home and family behind, is where his real spiritual journey starts.
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The sages explain that Abraham knew the Creator from age 3. After that, he dedicated his whole life to spreading the faith in the Creator. When Abraham reaches age 75, the Torah is asking him to leave and to go from his land, from his country, from his father's house, to get out of his comfort zone and just to go. All these years until age 75, the Torah is completely ignoring Avraham. I mean, he had a past before that, but it's like his life begins when the Creator is commanding him to leave and to step outside himself. Until that command, Avraham was just like any other person living, living at that time, but, but he was able with his intellect and understanding to know the Creator.