God called Abram out of circumstances that would have required what is, in our eyes, a great amount of faith. In Genesis 12, God basically told Abram to leave behind everything he has ever known and go to a place that he doesn't know. God promised to bless Abram if he did this. I imagine that this must have been a tough decision for Abram. After all, Abram was leaving behind his father, and had no idea if he would ever see him again. He also left behind a brother, apparently only a short time after his other brother died. The tendency would be to desire those things that are familiar; those things that Abram wouldn't have wanted to lose. Yet Abram decided to leave everything that was familiar to him in order to follow God and receive His promises.
God promised Abram, who was childless at the time, that he would become the father of a great nation. Abram was 75 years old and childless. His wife, Sarai, was 65 years old, and for her to have a child, it would have taken a miracle. This God who called Abram was claiming to be able to provide that miracle.
I can only imagine what was going through Abram's mind. What did Abram think? Did he wonder if this God that called him could even work such a miracle? Did he think that he was being lied to? Was he overjoyed that this God that called him was concerned with his needs? Did he wonder if his mind was merely playing a trick on him? Can you imagine what was going through Sarai's mind, too? I don't know what Abram or Sarai were thinking, but this calling of Abram surely had more impact on him than it does on us when we read the story.
Regardless of what was going through his head, Abram made the decision to believe what God told him. Abram left everything he had ever known because of God's calling. This was a difficult thing for Abram to do, no doubt. However, Abram made the decision to believe what God said, and to trust in His character, the He would not lead Abram astray. And He didn't.
Abram trusted that what God had told him was true. He did not simply tell God that he would go, then delay his travels. Rather, Abram showed God that he believed Him by doing what God commanded him to do. From the text of Genesis, it appears that Abram immediately responded to God's call.
Abram placed everything on the line to respond to God's call, and God was faithful to Abram. If you don't believe me, continue to read the story in Genesis. Not only did God bless Abram wherever he went, but God gave Abram favor in the eyes of many with whom Abram crossed paths. God not only gave Abram a son, but the Savior of the world came from Abram's lineage. Abram's name was changed from Abram to "Abraham", signifying his parental status that would not have come about if he had not left everything to follow God.
God was faithful to Abram, and He will be faithful to us. We must always act in faith, believing God and taking Him at His word. When we do this, we allow Him to do great things in our lives and in the lives of others. May our response be that of Abram, who believed God, and showed it by his actions.