Israel's encounter with God at Mount Sinai (Ex 19-20) — the foundational event of the OT covenant. Three days after the people arrived, the mountain was wrapped in smoke because the LORD descended upon it in fire (Ex 19:18); the whole mountain quaked greatly; the voice of the trumpet grew louder; God spoke the Ten Commandments from the cloud. The people stood at a distance, trembling, asking Moses to mediate. Sinai established the Mosaic covenant, the law, the tabernacle pattern, and the priesthood. Hebrews 12:18-24 contrasts Sinai with Zion: the believer comes not to the terrifying mountain that could not be touched but to mount Sion, and unto the city of the living God, the heavenly Jerusalem... and to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel. Sinai is the necessary backdrop against which the new covenant's grace is measured.