Chapter 15, verse 6: "And he believed in the Lord; and he counted it to him for righteousness."
God had just told Abram that he was going to have a child, and Abram, even though he was old, beleived Him. No questions.
Verses 13&14: God told Abram that his seed would be in a strange land for four hundred years. Then the fourth generation would be free to leave. He was telling him that his great grandchildren would be slaves in Egypt for four geneations.
The post for Matthew 15-19 will be up later.