Abraham was the one who believed God (where that belief was credited for righteousness). But when it comes to Abe's belief, it's pretty powerful! Let's see how powerful Abe's belief can be.
Contrary to hope, Abraham believed and had hope (4:18).
Despite the ridiculously old body of both Abe and Sarah, Abe "did not consider [their bodies] dead" (4:19).
Even with unbelief factored in, Abraham did not falter -- but he "was strengthened in faith" (4:20).
Abraham's faith was strong enough to stand in spite of insurmountable odds! But what made it that way. Well, Abraham did get various promises (including having many descendants), though natural odds for having children at a very old age is pretty low. Keyword on "natural" -- because Abraham looked to God. In particular, Abraham was "fully convinced that what He had promised He was also able to perform" (4:21). Considering that God is creator of the universe (and instigated the Big Bang), Abraham likely knew that God was more than able to produce offspring from Sarah in spite of natural old age. With a full conviction, "[Abe's faith] was accounted to him for righteousness" (4:22).
Today, Jesus provided a recovery route with the great resurrection feat (demonstrating that death can be countered) on behalf of us, even though we screwed up. Believing in Jesus as Lord and Savior for these is essentially a personal conviction that what Jesus did on behalf of YOU is true. And Jesus had performed such things. But why not stop there? Eventually, Paul will talk about how Jesus's great love, care, and security provision in Himself means we don't have to worry about fear -- and God has done some unexpected wonders even today.