Actually we must blame Augustine for this nonsense. He had many good things to say, but he also misinterpreted Scripture. In the end every Christian has a duty and obligation to carefully examine the doctrines of men in the light of Scripture. Sadly, dyed-in-the-wool Calvinists refuse to do so, and distort the plain meaning of many verses and passages, so that they can hang on to this False Gospel.
Why preach the Gospel if the outcome has already been predetermined by God? That is the position of Hyper-Calvinists, who are at least consistent with their beliefs.
The problem here is that we refuse to understand and accept that God is wiser than man. Just because you see inconsistency doesn't mean that there is inconsistency. The real problem here is not your refusal to accept that God is all powerful, it is your refusal to believe that you are powerless. Jesus came to seek and save, not seek and make saveable. We have Arminius to thank for this view of synergism, as it was he and pelagius who spread this heresy in the first place. I can understand it being hard to give up any power you thought you had, but it is necessary to understand that we, as men, have no power. Just because we cant understand something doesn't make it untrue!!
Why preach the Gospel?
1.) Christ told us to do so. No matter what, this is enough of a reason for me. Even if our evangelism does nothing, Christ told us to do something, so we ought to do it!
2.) This shows a lack of understanding the power of God. Maybe He chooses to use you to reach one of His chosen! What a glorious work! This also relieves the pressure on yourself... your performance in evangelism means nothing. We water, but God brings the increase of faith.
Man choosing ultimately retracts from God's glory. if man has something to add to His Salvation, or if it is simply making the 'right choice', then that hardly brings God glory; He didn't save you; without your permission it was impossible. No, He instead sits back, ringing his hands and hoping that maybe you will let Him save you.
Why preach the Gospel? Allow me to retort with this: If man chooses God, why pray for the lost souls of loved ones? If it is their choice, why ask God to do something you say He can't do?