I Samuel 17:36 Thy servant slew both the lion and the bear: and this uncircumcised Philistine shall be as one of them, seeing he hath defied the armies of the living God.
When the battle against Goliath was before David he wasn’t worried about whether God would work in his life or not. He knew that He would work. David knew that all of his prior victories had been the work of God. He didn’t think that he just killed those animals who threatened him and his sheep. He knew that God had used him. Now there was a giant in the way. The giant was threatening God’s people. He was defying God. David knew that God would give him strength to defeat this enemy.
You can know that God is going to use you. You can know that God is at work in your life. You can believe that.