Genesis 15:12 says, And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him.  Abram fell into a deep sleep and it says a horror of great darkness fell on him.  Matthew Henry says this was more than just a normal sleep, and that the horror came from not understanding it before God spoke to Abram.  We may at times be spiritually asleep and need to once again listen for God to speak to us.  When the fears of the world come, God will always see us through them if we listen to Him and follow His guidance.  Verse thirteen states, And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years;  God had told Abram that he was to be the father of a great nation, but God now told him that it was not going to be greatness in worldly terms.  Abram's descendants were to be strangers in a land that was not theirs and serve others for four hundred years.  When we accept Christ as our Savior and Lord, we are not promised worldly greatness, and we will always be strangers to this world.  We are simply passing through, and we are called to serve others.  Verse fourteen adds, And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance.  God told Abram that He would ultimately judge the nation that Abram's descendants served, and his descendants would come out with great substance.  One day, God is going to judge the world, and those that are His will come out with the riches of heaven, though that is not really what is important.  The important thing is that the followers of Christ will come out of God's judgment with an everlasting relationship with God with no obstacles between God and them.  Verse fifteen continues, And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age.  God promised Abram that he would die at peace at a good old age.  I believe that as followers of Christ we should always die at peace.  That does not mean that everything in the world will be right, but our relationship with God should always leave us at peace with leaving this old world.  Verse sixteen concludes, But in the fourth generation they shall come. hither again: for the iniquity of the Amorites is not yet full.  Though they would serve for awhile, Abram's descendants would have the land restored to them.  God's promises are always sure, but they are also on His timetable.  Verse seven declares, And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces.  These were just physical signs of God's presence.  Verse eighteen states,
In the same day the LORD made a covenant with Abram, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates:  Verse nineteen adds, The Kenites, and the Kenizzites, and the Kadmonites,  Verse twenty continues, And the Hittites, and the Perizzites, and the Rephaims,  Verse twenty-one concludes, And the Amorites, and the Canaanites, and the Girgashites, and the Jebusites.  God promised Abram a land, but He never said that there wouldn't be others there who opposed them.  When we accept Christ, we are a part of God's kingdom, but in this world there will always be those who oppose us.