Moses said, "But I will be with you, and this shall be the sign for you, that I have sent you: when you have brought the people out of Egypt, you shall serve God on this mountain." Exodus 3:12
How did Moses know that God had raised Him up & called Him to save Israel from Egypt as the human leader of God's Covenant People? It was neither an insignificant nor inconsequential sign - that of holding a service of worship at Mount Sinai! It was by the LORD appearing in the earthquake, fire, smoke, thunder & darkness at Horeb, with the voice that boomed in the heavens while a consecrated church bowed in trembling at the foot of the hilltop, Holy of Holy, sight - and because, along with seventy elders, Moses was caught up into the clouds to worship & eat in before the sapphire pavement of the presence of the LORD; and finally, on account of the fact, that after that we went up higher, leaving Joshua behind, into the nearer presence of the revelation of the Glory of the LORD (and even later when the Shekinah Cloud of God condescended into the Tent). After this great & mind-exploding experience of divine light, neither Moses or Israel were left with any doubt - Moses had been appointed as God's mediator of the Sinaitic Covenant.
How can we be sure the Gospel is really true? Do we not know for certain that God has appointed Jesus as the New Covenant Mediator that has been sealed in His shed blood? Have we not seen with heart-eyes the dove descend from Heaven and rest and remain on Christ as He identifies Himself with, & to be the One appointed to wash, the pollution of our sinful filth, while spotlessly innocent Himself? Have we not bowed to behold the brightness of His face, as no Word remains except the Glory of God's Son in whom the Father is well-pleased? Do we not regularly come to gasp at the majesty of the Cross where the Good Shepherd lays down His life for His wayward sheep, and as the unblemished sacrifice is bruised, cursed & pierced, once-for-all, to definitively atone for all believing, penitent, sinners of God's flock?
...have we not come to a more joyful, glorious, Mountain - the heavenly Jerusalem, Mount Zion, on which God-Alive sets His love? To millions of angelic cousins assembled in bright garments for a feast, and the church of the firstborn whose names are carved in blood in the assembly hall of God? To the Lord of all to whom we give account, and to a company of righteous-by-faith, now completely spotless, glorified, saints? A better, brighter, more "Bow-in-worship Mountain"!
We now know for sure, as we recall His sprinkled blood, that the Lamb of Calvary, whom we behold in faith, is the sole, chosen, exalted Mediator of the New Covenant - His bloody benediction pronounces peace & life, and every spiritual blessing upon saints - for He has killed our curse, in the death of death, in the sacrifice of Christ.
But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, and to innumerable angels in festal gathering, and to the assembly of the firstborn who are enrolled in heaven, and to God, the judge of all, and to the spirits of the righteous made perfect, and to Jesus, the mediator of a new covenant, and to the sprinkled blood that speaks a better word than the blood of Abel. Hebrews 12:22-24
Behold, come, bow, worship at this Mountain in full assurance of faith.