From what we are told, Socrates was accused and condemned of atheism. He denied the accusation but accepted to verdict of Death, instead of taking advantage from the opportunity to leave Athens and live elsewhere.
Socrates main philosophy and teaching was skepticism and/or agnosticism: "I only know that I don't know nothing", but it also seems he has strong ethical convictions and he was not afraid to die. Socrates attributed his ethics (and denial of charges of atheism) to what he called Daimon, a voice of conscience that told him what not to do, when he was about to do something unethical, but not what to do. We don't know how exactly Socrates experienced his Daimon, as auditory hallucination or other form of "imaginary friend", or something else.
He questioned the beliefs of his fellow Athenians, the rational basis behind them, for which he was accused and condemned of corrupting the youth of Athens. He didn't offer other belief systems to replace those he torn down, but he encouraged critical thinking, ethical behavior and search for virtue.