John Gill Commentary Romans 4:18

"Who in hope believed against hope, to the end that he might become a father of many nations, according to that which had been spoken, So shall thy seed be." — Romans 4:18 (ASV)

Who against hope believed in hope
Abraham believed the promise of God,

that he might become the father of many nations ,
being assisted by a supernatural aid: "in hope"; of the fulfilment of it by the grace and power of God:

"against hope": against all visible, rational grounds of hope; Sarah's womb and his own body being dead, but inasmuch as God had said it, he believed:

according to that which is spoken, so shall thy seed be ;
his faith rested upon the word of God, which showed the nature of it, and that it was of the right kind.