Romans 4:17 — (As it is written, I have made thee a father of many nations,) before Him whom he believed, even God, who quickeneth the dead, and calleth those things which be not as though they were.
Father — Genesis 17:5 — Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee. It says "I have" even though it hadn't yet occurred because God's promise made it sure.
quickeneth (gives life to) the dead — in this case, Sarah and Abraham's bodies were dead to child-bearing but God made them alive.
before = in the presence of