Without weakening in his faith, he faced the fact that his body was as good as dead since he was about a hundred years old and that Sarah’s womb was also dead. – Romans 4:19
Abraham had amazing faith. However, he was not a man who always had faith at every moment; he grew into his faith and wrestled with doubt many times along the way. Sometime after God told Abraham twice that he would give him children, Abraham voiced his doubt to the Lord for the first time (Gen 15: 2-3) and the Lord reassured him; when Abraham believed God, it was credited to him as righteousness. After this, when God appeared to him again for the third time to confirm his promise, Abraham laughed incredulously (Gen 17:17). Even after all this, Paul can truthfully declare that Abraham believed God without weakening in faith because faith is not the absence of doubt or the ignorance of reality. Doubts will come and try to destroy our faith. However, it is up to us to choose to either entertain it so it can eat our faith alive or banish it.