Why is Abraham called the father of all nations?
God chose Abraham that through him all the nations on earth might find blessing
Abraham was Sarah's husband. Abraham was also the father of the Jewish nation. Additional Comment: Abraham is the father of all Israelites - physical and spiritual. They are many nations/peoples. Genesis 17:5 New International Version (NIV) 5 No longer will you be called Abram; your name will be Abraham, for I have made you a father of many nations.
God chose Abraham to the the father of the Israelite nation. Through Abraham, came his son Issac and then his son Jacob who in turn had twelve sons who formed the twelve tribes of Israel. These twelve tribes and their descendants eventually populated most of the world. A great number of people from all nationalities can trace their family roots all the way back to Abraham.
Genesis 17 has the answer to the name changes. Abram meant "High Father'. Under the covenant God made with Abram "all nations of the earth would be blessed. God changed Abram's name to Abraham as he would be the "Father of many nations", the meaning of Abraham. Sarai meant "My princess". Sarai would become the ancestress of the promised nations and kings. Sarah means "Mother of Nations."