The very earliest humans DID know God, deeply and personally. Then they rebelled against him, fell from his holy and sustaining grace, and forsook him to follow after their own evil ways. Having forsaken God, the source of all good, humans were turned over to pure evil and foolishness.
In our evil and foolishness we invented "gods" after our own sin-tainted image, to satisfy our God-given need for worship within the context of an inherently evil nature. The real God made us in his image, but we rebelled against him, willingly suppressed the truth in unrighteousness, and invented "gods" in our own image. Ultimately all religion is idolatry. Christianity is not religion per say, its a real relationship with the one true and living God.
So to answer your question directly, God did reveal himself to us from the beginning. But we walked away from him. So he chose to reveal himself to and through the nation of Israel, and through this people he brought forth his Son, the Messiah, to bear the sins of mankind and suffer condemnation in our place. In so doing all can be reconciled to God. Even though we rejected him, hated him, and invented "gods" of our own to worship, he was so gracious that he poured out our just penalty upon his own son that we could be reconciled, if only we'd repent and believe in him. God is hardly unjust in his dealings with us, he is eminently and unfathomably merciful and gracious.