When I read Exodus 20:21 where it said that Moses " drew near unto the thick darkness where God was." I thought to myself, does God dwell in darkness? God is light and truth, no darkness in him. I had to go back to Exodus 19 and read more. God appeared in a cloud, the glory cloud of God. I have seen dark clouds, full of rain; then there was fire on the mountain and smoke. In the natural I thought of a volcano with dense smoke, but this was more than a natural occurrence. God was there, in the darkness. All the people feared at his presence and would not come near because they were unclean and lacking in faith. Moses went into the cloud and the darkness of smoke on the mountain to speak with the angel of God.
Exo 19:9 And the LORD said unto Moses, Lo, I come unto thee in a thick cloud, that the people may hear when I speak with thee, and believe thee for ever. And Moses told the words of the people unto the LORD.
Exo 19:10 And the LORD said unto Moses, Go unto the people, and sanctify them to day and to morrow, and let them wash their clothes,
Exo 19:11 And be ready against the third day: for the third day the LORD will come down in the sight of all the people upon mount Sinai.
Exo 19:12 And thou shalt set bounds unto the people round about, saying, Take heed to yourselves, that ye go not up into the mount, or touch the border of it: whosoever toucheth the mount shall be surely put to death:
Exo 19:13 There shall not an hand touch it, but he shall surely be stoned, or shot through; whether it be beast or man, it shall not live: when the trumpet soundeth long, they shall come up to the mount.
Exo 19:14 And Moses went down from the mount unto the people, and sanctified the people; and they washed their clothes.
Exo 19:15 And he said unto the people, Be ready against the third day: come not at your wives.
Exo 19:16 And it came to pass on the third day in the morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of the trumpet exceeding loud; so that all the people that was in the camp trembled.
Exo 19:17 And Moses brought forth the people out of the camp to meet with God; and they stood at the nether part of the mount.
Exo 19:18 And mount Sinai was altogether on a smoke, because the LORD descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.
Exo 19:19 And when the voice of the trumpet sounded long, and waxed louder and louder, Moses spake, and God answered him by a voice.
Exo 19:20 And the LORD came down upon mount Sinai, on the top of the mount: and the LORD called Moses up to the top of the mount; and Moses went up.
Exo 19:21 And the LORD said unto Moses, Go down, charge the people, lest they break through unto the LORD to gaze, and many of them perish.
Exo 20:18 And all the people saw the thunderings, and the lightnings, and the noise of the trumpet, and the mountain smoking: and when the people saw it, they removed, and stood afar off.
Exo 20:19 And they said unto Moses, Speak thou with us, and we will hear: but let not God speak with us, lest we die.
Exo 20:20 And Moses said unto the people, Fear not: for God is come to prove you, and that his fear may be before your faces, that ye sin not.
Exo 20:21 And the people stood afar off, and Moses drew near unto the thick darkness where God was.
The people were not able to touch the holy mountain when God's presence was there. Moses had gone up and he had received the law. Moses spoke to the people that God was proving or testing them, for they had little faith. Because they were lacking in faith God gave Moses laws to guide them until faith was formed in their hearts. As believers by faith in Jesus Christ we will be tested. Darkness will come to us, but God is in the darkness to show us something. He is there with us to bring us through. Sanctify yourselves, cleanse yourself by confessing your sins and stay close to the presence of God. It is a good thing to fear God, but by being in Jesus we may come boldly into his presence.
Heb 4:14 Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.
Heb 4:15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
Heb 4:16 Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need.