In Genesis 15 God promised Abraham that his descendants would inhabit all the land from the border of Egypt to the Euphrates River. In Genesis 17 he promised that his descendants would inhabit the land of Canaan only. So what happened to the original promise?
The explanation is simple. Abraham had two sons, an illegitimate son Ishmael who was born to his wife's handmaid Hagar, and later a legitimate son by his wife Sarah. Ishmael was the ancestor of the Arab race, and Isaac the ancestor of the people now known as Jews. So in chapter 15 God was speaking about all Abraham’s descendants, including those of Ishmael, but in chapter 17 it is evident that he was speaking about Isaac's descendants only, who were to inhabit the area of land now known as Israel. If Abraham had waited for God to fulfil his promise of a legitimate son the Arab race would not have been born and Isaac's descendants would have inhabited all the original land area which now belongs mainly to Arabs, just as God originally promised.
Abraham was not always particularly obedient or faithful to God. He took his nephew Lot with him into Canaan when God had told him to leave all his family behind in Haran. He emigrated to Egypt during a famine when God had told him to stay in Canaan. He lied about his wife Sarah being his sister and let the pharaoh take her as his wife. As I wrote above, he fathered a child by his wife's maid, and when God later reassured him that Sarah would give birth to a son in their old age he laughed in disbelief. Finally when Sarah eventually did give birth, Abraham allowed her to dispatch Hagar and her young son into the wilderness to fend for themselves.
But at other times Abraham was obedient. He did leave Haran for Canaan as God had told him to; he originally believed God's promise that he would be the father of a great nation; and when God told him to circumcise all the males in his household he did it the very next day. Eventually he demonstrated the most amazing obedience and faith in being willing to sacrifice Isaac at God's instruction, not knowing that God was simply testing him and would provide a substitute sacrifice in the form of a ram caught up in a nearby bush.
So it seems to me that God did not choose Abraham because he was initially a particularly righteous man. And God did not abandon him when he doubted and disobeyed him. Most amazing of all, God allowed him to make what was probably the second worst mistake in human history. In fathering Ishmael, Abraham was responsible for producing millennia of conflict between Jews and Arabs, as well as the internal conflicts in the Arab world and more recently the rise of Isis and Muslim terrorism. Abraham was the cause of it all!
What does this story teach us? When God calls us to come to him through faith in his Son Jesus, it is not because we are more righteous than anyone else. We may be disobedient and at times not believe what he says, and sometimes we may make dreadful mistakes which have very bad results. But the God who calls us remains remains faithful to his promises and he can still fulfil them, even though it may not be possible to fulfil them as completely as it might otherwise have been. For our part we can grow into place of faith and obedience, as Abraham showed by circumcising his household at God's instruction and later by being willing to sacrifice his beloved son Isaac. In fact the New Testament letter to the Hebrews says that Abraham believed God would somehow bring Isaac back from the dead! In the end he fathered the race that brought Jesus Christ into the world; Jesus who was indeed sacrificed for our sins and who did return to life in order to save everyone who believes in him from final death.
From Abraham's experience we can see how our faith and obedience to God can increase over the years, and how he can produce something great and lasting out of our life even if our trust in him is initially weak and faltering and we make mistakes along the way.