"As the sun was setting, Abram fell into a deep sleep, and a thick and dreadful darkness came over him. Then the Lord said to him, 'Know for certain that your descendants will be strangers in a country not their own, and they will be enslaved and mistreated four hundred years. But I will punish the nation they serve as slaves, and afterward they will come out with great possessions. You, however, will go to your fathers in peace and be buried at a good old age. In the fourth generation your descendants will come back here, for the sin of the Amorites has not yet reached its full measure.'" NIV translation
Abram had just received promises from God and yet we see, here, that "a thick dreadful darkness came over him". What is this darkness? This is fear and doubt which is sin and we see that it was in his mind as he fell asleep. While he was in this darkness, God spoke to him and reassured him of the promise and gave him details of what was to come. We are reminded that, throughout the Bible, the number four is associated with earthly trials. In the details that God gave him, we see that Abram's people were going to face the great trial of slavery but, in the end, they would be drawn to God. This same pattern of trials being used to bring people to God is repeated over and over throughout the Bible. This is also the basis for the period of time that is referred to as the Great Tribulation and is described in Revelation. Just as God spoke to Abram during his darkness of doubt, He calls to us in our times of trials. He calls us back to the promises that are contained in His word through Jesus Christ.