It was not the first day that David got his sling that he took down Goliath. He had used it many times on lesser beings, on animals in the bush. He not only mastered the sling, the sling also mastered him, learning to ‘feed’ off the anointing of God on his life for its effectiveness. It was not because Saul’s body armour was heavy, it was because it belonged to Saul. David had not trained himself on how to channel God’s power through it, he had not ‘tested’ it.
What is your talent? What is your skill? What is your dream? Start putting it to the test NOW! Start learning how to power that natural thing with supernatural strength NOW! Work now while it is day, for night is coming when you cannot work. On the physical plain, your dream depends on opportunities to grow, if you do not prepare for the opportunities, they will pass you by. According to Jesus’s parable, be ready for when the master of the house knocks.
Then Saul clothed David with his armor. He put a helmet of bronze on his head and clothed him with a coat of mail, and David strapped his sword over his armor. And he tried in vain to go, for he had not tested them. Then David said to Saul, “I cannot go with these, for I have not tested them.” So David put them off. (1 Samuel 17:38, 39 ESV)