And being fully persuaded that, what he had promised, he was able also to perform.
Note 13 at Ro 4:21: Notice that Abraham wasn't just persuaded; he was fully persuaded. Many people have been persuaded that the promises of God are true, but they stop short of meditating on God's Word until they become fully persuaded. Strong faith belongs to those who continue in God's Word until all doubt is removed.
Note 14 at Ro 4:21: This is so obvious that it should go without saying, but the truth is, we really do doubt that God can perform His promises to us. How could this be? The answer lies in the way God made our hearts.
Whatever we focus our attention on is what our hearts will believe, and whatever we neglect is what our hearts will disbelieve (see note 10 at Mr 6:52). If we allow ourselves to meditate on our problems and all the reasons it looks impossible for God to move in our situations, then we will believe that our problems are bigger than God. However, when we keep our minds stayed on the promises of God, nothing is too difficult for Him (Jer 32:17 and 27).