The prophet Joel called him "the hidden one," and the sages took the phrase at its full weight. "I will remove far from you the hidden one, and I will drive him into a land barren and desolate" (Joel 2:20). That hidden one, the rabbis taught, is the yetzer hara, the evil inclination that keeps itself out of sight in the human heart.
The verse continues: "with his face toward the former sea, and his hinder part toward the latter sea." The sages read each direction as a Temple. The face, looking backward, is fixed on the First Temple, which the tempter destroyed by corrupting those who served in it and cutting down the disciples of the wise who studied there. The back, turned forward, is fixed on the Second Temple, which he destroyed the same way, by the same slow work.
This is why Ezekiel records the divine promise, "I will take away the heart of stone out of your flesh, and I will give you a heart of flesh" (Ezekiel 36:26). The stone is not an ornament. It is the tempter's handiwork, hardened over years of being fed. The heart of flesh is what remains when the hidden one is finally driven off (Sukkah 52a).
The rabbis were blunt about the lesson. Enemies with armies destroy a sanctuary once. The enemy inside destroys it twice, and can destroy it again.