Exodus 20:19, 21 And they said unto Moses, Speak thou with us, and we will hear: but let not God speak with us, lest we die…And the people stood afar off, and Moses drew near unto the thick darkness where God was.
The supernatural manifestation of the Presence of God at Mt. Sinai was terrifying and awesome. The people saw the thunderings, the lightenings, the trumpet, and the smoking, and they stood afar off. The all-powerful presence of the Holy God is much too terrifying for sinful man to abide, for fear of death, since sin is the cause of death. If Almighty God should come and speak directly with us we know that we would be consumed. So we fear. And rightly so.
Job pleaded for 38 chapters to have God speak with him so he could defend his innocence. And when God did finally speak to him, he was bowled over and overwhelmed by the awareness of his sinfulness. His only response was to repent in dust and ashes. So he did and was restored. Job learned terror and was saved.
The people who were stricken with fear by the magnificence of God begged Moses to speak with them instead of God directly. They wanted Moses to mediate, come between, God and sinners. So Moses, as priest and prophet, went into the thick darkness where God was and received the Ten Words. Later he would speak with God face to face, and when he came out of the tent even the secondary reflected glory on his face was overwhelming.
God granted their prayer and Moses became the Mediator between Man the Sinful and God the Holy. God has to mediate His thoughts, mitigate His words, soften His majestic power, and filter His strong love through means. The means of grace for us are Word and Sacraments. It looks and sounds earthly, normal, natural, and nothing spectacular, but behind it is the awesome power and overwhelming love of God. Plain human beings mediate the Gospel to us, so it doesn’t feel as powerful as it is. And also, humans crucified the Son of God for he mediated God and looked human.
We should not denigrate the human instruments and natural elements through which God comes to us, since we asked for a mediator.