A very famous passage in the Talmud (Hagigah 14b) tells us that four sages entered Pardes: Ben Azai, Ben Zoma, Aher, and Akiba. All four were very important and well-known students of Torah.
When Shimon Ben Azai entered Pardes, he died. Ben Azai was famously pious, studying Torah day and night; and his death was a tragedy. Elsewhere in the Talmud (Sotah 49a) we are told that so great was he as a student of Torah that diligent study of Torah died with him and in another place (Berakhot 57b) that he was so pious that even seeing him in a dream is a sign that you yourself will become more pious. Precious in the eyes of the Lord is the death of his saints.
When Shimon Ben Zoma entered Pardes, he became insane. Ben Zoma was famous for precise, well-balanced interpretation of Torah. In another place (Berakhot 57b) the Talmud tells us that he was so wise that even seeing him in a dream is a sign that you yourself will become more wise. Finding honey, he tried to eat more than his fill.
When Elisha Ben Abuya Aher entered Pardes, he began 'to cut the shoots,' which is a Talmudic idiom for teaching heresy. Aher entered Pardes and lost his way; more than that, he led others astray. Rabbinical tradition suggests that his heresy was believing that entering Pardes gave him license to ignore Torah and, worse, that because only God truly was, all things, even evil actions, were simply God Himself. Others suggest that he came to believe that there were two Gods. But Aher had not been a lesser rabbi than the others; indeed, he was regarded as greater still than either of the other two. But he let words take the place of truth.
So the first three of the four who entered Pardes. But the last, Rabbi Akiba, entered Pardes and left it in peace. Akiba himself would have denied that he was in any way superior to his companions. He had been an illiterate shepherd who only began to study Torah because the woman he loved insisted that he do so. And despite the fact that he was a good man, there is no reason to think that he was morally better than the others. So what was the difference that made it so that Akiba could enter and leave Pardes without harm? Some say it is because he alone had made the effort before entering Pardes to leave signs that would lead him back. Others say it is because he was more steeped in Halakhah. But I wonder if it really was that he alone of the four was truly a rabbi: drawn to God by his love of God, he taught others and drew them along behind him. Thus he did what a teacher does: having learned great things, he returned to teach them. Because he still had students to teach, he could not simply give up his life like Ben Azai; because he taught, he moderated what he learned to what he could teach, and thus did not overfill his mind like Ben Zoma; because he truly taught Torah, he would not allow himself the license to ignore it in favor of following his thoughts wherever he pleased, as Aher did. Once past the wall, only the one who loves to teach returns safely.
This post is due in part to Arsen Darnay's meditation on paradise here.