He did not weaken in faith when he considered his own body, which was as good as dead (since he was about a hundred years old), or when he considered the barrenness of Sarah's womb. (ESV)
What are the things that would cause you to weaken in faith? Perhaps you have been trying to advance yourself in a particular career, but as the years go by you begin to wonder if you will ever get that position you feel that you should have. Maybe, like Abraham, you would like to be a father and it seems like it will never happen.
Consider Abraham for a moment. The odds were not exactly in his favor. Can you imagine if he were alive today. If he went into a doctor's office and told his physician that Sarah was barren and that he was one hundred years old what do you think his doctor would say? "Sorry Abe, this is just not a human possibility."
Maybe you have been told by someone who is an expert in their field that what you are desiring for your future is ridiculous. We need to be reminded that there are things that are impossible for man, but nothing is impossible with God. Abraham's circumstances would suggest that fatherhood was not an option for him.
Abraham is an example for all of us that our focus should be on placing our faith in God no matter what the circumstances look like. If you have been given certain spiritual gifts, a calling on your life, and physical abilities and talents, you should know that God wants to use them for his glory. He will accomplish what he has promised to you. He is faithful. He does the impossible. He is God that way.
Do not weaken in your faith. Consider Abraham. He believed the impossible and God made it possible.