Packer writes in his classic work, Evangelism and The Sovereignty of God:
If we regarded it as our job, not simply to present Christ, but actually to produce converts – to evangelize, not only faithfully, but also successfully – our approach to evangelism would become pragmatic and calculating… This shows the danger of forgetting the practical implications of God’s sovereignty. It is right to recognize our responsibility to engage in aggressive evangelism. It is right to desire the conversion of unbelievers… But it is not right when we take it on us to do more than God has given us to do. It is not right when we regard ourselves as responsible for securing converts, and look to our own enterprise and techniques to accomplish what only God can accomplish. To do that is to intrude ourselves into the office of the Holy Ghost, and to exalt ourselves as agents of the new birth. And the point that we must see is this: only by letting our knowledge of God’s sovereignty control the way in which we plan, and pray, and work in His service, can we avoid becoming guilty of this fault. For where we are not consciously relying on God, there we shall inevitably be found relying on ourselves, (pg. 27-29).