You can know for sure that God will complete a good work in you because Jesus has already made a down payment on your life. There is nothing that you can do to redeem yourself. Let pastor David's message be an encouragement for when you stumble or fail. You don't have to do it all on your own because Jesus has already paid the penalty for your sin. You only need to come to him asking for forgiveness. Your salvation lies in what Jesus did on the cross and not on your actions.