The Torah states, "Wherever I shall mention My name, I will come to you and bless you" (Exodus 20:21). The Mekhilta interprets this verse with a startling specificity: "where I am revealed to you" means the Temple in Jerusalem. God's name is mentioned, fully and explicitly, only in the place where God chooses to manifest the divine presence.
From this verse, the sages derived a far-reaching legal ruling: it is forbidden to pronounce the Shem HaMeforash (שם המפורש), the explicit four-letter Name of God, the Tetragrammaton, anywhere outside the Temple. "In the borders," meaning in all the territory beyond the Temple precincts, the Name must not be uttered.
This prohibition shaped Jewish practice for millennia. Inside the Temple, the High Priest would pronounce the Tetragrammaton on Yom Kippur, and the assembled worshippers would fall on their faces upon hearing it. But outside those walls, Jews substituted other names: Adonai, HaShem, the Holy One Blessed Be He. The true Name was too sacred for ordinary space.
The Mekhilta's reading transforms the verse from a general promise into a legal boundary. God is saying: I will reveal My name in one place, and only in that place may it be spoken. The rest of the world is "the borders," territory where the full divine Name remains hidden. After the Temple's destruction, the pronunciation of the Tetragrammaton was lost entirely, fulfilling in a tragic way the principle that God's explicit Name belongs only to the place of God's choosing.