Why Ishmael is Sent Away.

In Genesis 21 we read about Ishmael, the son of the slave Hagar, mocked Isaac, the son of Sarah. I have always thought it was cruel for Sarah and Abraham to banish Hagar and Ishmael. But after reading the commentaries I now understand why. Ishmael had been the first-born. The inheritance from Abraham would have all been his until Isaac was born. This seemed to make Ishmael angry - so he mocked the child, and as Paul said, he persecuted Isaac. Ishmael was 17 years old at the time and could have been a threat.

Gill's Exposition of the Entire Bible
mocking; either at the entertainment made at the weaning of Isaac; or rather at Isaac himself, laughing at his name, and treating him with contempt as his younger brother, and boasting that he was the firstborn, and that the inheritance belonged to him; and threatening what he would do to him, should he hereafter offer to dispute it with him, under pretence of the promise of God that he should be Abraham's heir, and at which promise also he may be supposed to mock: and that this contention was about the inheritance seems plain from the words of Sarah inGenesis 21:10; and in it Ishmael might not only rise to high words, but come to blows, and beat his brother; for it is observed the word used sometimes so signifies, 2 Samuel 2:14; wherefore the apostle might truly call it a persecution, Galatians 4:29; and as even cruel mockings are, Hebrews 11:35. http://bible.cc/genesis/21-9.htm

I also used to wonder why Abraham didn't make sure Hagar and Ishmael had enough water and food to go on their trip. But the commentaries said they had enough but they got lost in the desert. When the water and food was gone and they were dying, it was then that Hagar prayed. God appeared immediately and gave her more than she prayed for. He saved their lives and promised to be with them.

"God heard the boy crying, and the angel of God called to Hagar from heaven and said to her, "What is the matter, Hagar? Do not be afraid; God has heard the boy crying as he lies there. Lift the boy up and take him by the hand, for I will make him into a great nation." Then God opened her eyes and she saw a well of water. So she went and filled the skin with water and gave the boy a drink. God was with the boy as he grew up. He lived in the desert and became an archer. While he was living in the Desert of Paran, his mother got a wife for him from Egypt." Genesis 21:17-21