I believe the Great Commission would be impossible apart from the doctrine of unconditional election. If the winning of souls were entirely up to us, we would undoubtedly fail. The only reason any evangelist has won someone to Christ is because Christ has already gone into the world ahead of him. Or, to put it in Star Trek terms, Christ has boldly gone where no evangelist has gone before.
The God who has determined the end has also determined the means, and that includes believers going into all the world to proclaim the gospel. God has said that his word "shall not return to me empty, but it shall accomplish that which I purpose, and shall succeed in the thing for which I sent it" (Isaiah 55:11). That is encouraging news for every believer.