God offered Abraham something that seemed impossible - to become a father at almost a hundred years old, with a wife, Sarah, of the same years. Knowing that that isn't physically possible, Abraham believed. He had faith and didn't doubt for a moment, because God had said it would be. Today's reading from Paul's letter to the Romans (Rom 4:18-25) emphasizes this - it was Abraham's faith in God that "was credited to him as righteousness".
That is just what Matthew does in the Gospel reading (Mt 9:9-13), as well. He just got up from the customs post and followed Jesus. He took no time to consider his options, to weigh things out - even to pack; he just got up and followed Jesus. He had absolute faith in God - that he was being called by God to follow a new path.
What path does God call each of us to? Do you have the faith to pick up and follow, without a doubt in God, even if it seems impossible?