As you mentioned, the key is really about trust. Abraham learned to really trust God to do what He says He's going to do. God for years had already protected Abraham and blessed him. God had also promised Abraham would have a son even though his wife Sarah was well past child bearing age. Abraham saw God bring Sarah's womb back to life so that she may carry a child in her old age. It was because of the relationship Abraham had formed with his God that he trusted God to do all that he promised, including having large family of descendants from his son Isaac. Abraham trusted God to even bring Isaac back to life if he killed him in order to fulfill the prior promises.
Jehovah then appeared to Aʹbram and said: “To your offspring*+ I am going to give this land.”+ So he built an altar there to Jehovah, who had appeared to him.
all the land that you see, I will give to you and your offspring* as a lasting possession.+ 16 And I will make your offspring* like the dust particles of the earth, so that if anyone could count the dust particles of the earth, then your offspring* could be counted.
Abraham and Sarah were old, being advanced in years.+ Sarah was past the age of childbearing.*
By faith Abraham, when he was tested,+ as good as offered up Isaac—the man who had gladly received the promises attempted to offer up his only-begotten son18 although it had been said to him: “What will be called your offspring will be through Isaac.”+ 19 But he reasoned that God was able to raise him up even from the dead,...