Summary: By beginning with God’s appearance to Abraham he indicates that he doesn’t disagree with the fathers on what constitutes true religion—the religion of the Jews. For them, all religion, the worship of God, the doctrine of the law, all prophecies....
The God of glory appeared unto our father Abraham.
The God of glory is a Hebrew form of expression denoting "the glorious God." It expresses His "majesty, or splendor, or magnificence"; and the word "glory" is often used to describe the marvelous appearances in which God has shown Himself to people, (Deuteronomy 5:24—also see Exodus 33:18; Exodus 16:7, Exodus 16:10; Leviticus 9:23; Numbers 14:10). Perhaps Stephen meant to assert that God appeared to Abraham in some glorious manifestation, so that there would be no doubt that he was being addressed by Almighty God. Furthermore, the word "glory" may have been meant to ward off the charge of "blasphemy" against God, and to show that he regarded Him as being worthy of honor and praise.
By beginning with God’s appearance to Abraham he indicates that he doesn’t disagree with the fathers on what constitutes true religion—the religion of the Jews. For them, all religion, the worship of God, the doctrine of the law, all prophecies, depended upon that covenant which God made with Abraham; therefore, when Stephen declared that God appeared to Abraham, he embraced the law and the prophets, which stem from that first revelation.The God of Glory would also serve to distinguish Him from the false gods worshipped by the heathens.
The first call that Abraham received from God is not expressly recorded in Genesis, however, it is clearly implied in Genesis 15:7: “He also said to him, ‘I am the LORD, who brought you out of Ur of the Chaldeans to give you this land to take possession of it’” and Nehemiah 9:7; “You are the LORD God, who chose Abram and brought him out of Ur of the Chaldeans and named him Abraham.” The God of glory appeared to Abraham while he was in Mesopotamia. Abraham's childhood home was at Ur of the Chaldees in Mesopotamia, the country between the Euphrates and Tigris rivers. The second call is recorded in Genesis 12:1, but it doesn’t say specifically that God appeared to him, only that “The LORD had said to Abram.” He gave Abraham this second call at Haran, or Charran (the same), but Stephen declares that the family had gone from Ur to Charran, because of an earlier call (Acts 7:3, 4). Charran was on the route to Canaan, and Abraham made a stop there of five years, until his father died (Genesis 11:31, 32).
When he was in Mesopotamia.
God appeared to Abraham "when he was in Mesopotamia", a country that lay between the Tigris and Euphrates rivers. The Scriptures are silent on this appearance of God to Abraham, which Stephen has mentioned here, and the Jewish writers seem to hint at it, when they say, "thus said the holy blessed God to Abraham, as thou hast enlightened for me Mesopotamia and its companions, come and give light before me in the land of Israel.'' When Abram was in Mesopotamia (Genesis 11:3110), it is said that Abraham dwelt "in Ur of the Chaldees." The word "Mesopotamia" is the name given to the region between the Euphrates and the Tigris rivers. The name is Greek, and the region had other names before the Greek name was given to it. In Genesis 11:3110 and Genesis 15:7, it is called Ur of the Chaldees. Mesopotamia and Chaldea might not exactly coincide; but it is evident that Stephen meant to say that "Ur" was in the country afterward called Mesopotamia. Its precise location is unknown, but one theory says Ur of the Chaldees was situated near to Babel, and among the rivers, (Tigris and Euphrates), which gave the name of Mesopotamia to the country. When Steven says in Acts 7:4 that Abraham came out of Chaldea, it is evident that Mesopotamia contained Chaldea. After Abram left Ur he went Haran, or Charran where he received a second call (the same), but Stephen declares that the family had gone from Ur to Charran, because of an earlier call (Ac 7:3, 4). Charran was on the route to Canaan, and Abraham made a stop there of five years, until his father died (Ge 11:31, 3210).