The King James Version of this verse says Abraham was “fully persuaded” (v. 21). When you are dealing with something as impossible as having a child at the age of one hundred, your faith must be rock-solid! Abraham faced the fact that his body was dead (Romans 4:19), but he did not allow that consideration to weaken his faith. He came to a point in which his beliefs did not waver and he was “fully persuaded.”
When confronted with an impossibility, you must face the facts with the truth, which is embodied in the promises of God. As you spend time meditating on the promises of God, you will become “fully persuaded.” His promises will so saturate your mind and thinking that you will no longer waver in doubt when looking at outward circumstances. Rather, your heart will be convinced and remain fixed, regardless of what your mind thinks. Your heart will overflow with a faith that knows God has the power to do what He has promised. At that point, nothing the devil can do or say will move you from your position of faith.
Keep reading, studying, and digging into God’s promises. Persist in tearing down any area of unbelief, until you are “fully persuaded.”