Do we only do something if there is a guarantee of success? If so, then we will never reach our full potential. David had faith in God and trusted God but that didn't mean that he was guaranteed to win. Maybe God had another plan for David. Maybe God felt that David wasn't ready to win or that losing would be better for David and for others. God only knows the big picture and we need to trust Him. In this case, God wanted David to win so that EVERYONE would know that God existed and that if we fully trust God, He will have our back. Because no way in heck would that little guy kill a giant. It is a scientific fact.
Lord, thank You for examples in the Bible but more importantly examples in my own life where You show up and allow me to accomplish more than I ever could because of Your presence. And because I am just a puny little weakling like David.