And G‑d revealed Himself to him [Abraham]… (18:1)

When Rabbi Sholom DovBer of Lubavitch was a child of four or five, he entered into the room of his grandfather, Rabbi Menachem Mendel, and burst into tears. His teacher in cheder had taught the verse “And G‑d revealed himself to Abraham…” “Why,” wept the child, “doesn’t G‑d reveal Himself to me?!”

Rabbi Menachem Mendel replied: "When a Jew, a tzaddik,1 realizes at the age of 99 that he must circumcise himself, that he must continue to perfect himself, he is worthy that G‑d should reveal Himself to him."