Abram, now old and beyond the biological age of being capable to father a child, asks god for a sign, a means of helping him know that God will fulfill his promises. God reinforces his promise to Abram with a stellar object lesson. And Abram believed God. This belief was credited to him as righteousness. Likewise for Christians today, our 'work' is to believe God; to believe in the One He has sent, Jesus the Christ and it will be counted to us as righteousness.