Paul asks in Corinthians "But you know who made you to differ..." He implies that we know that it was God who made those who are saved to differ from the unsaved around us. But this creates a dilemma for the Arminian who agree with the Calvinists that man is totally depraved. But the Arminians teach that God gives every human being in the world a measure of "prevenient grace", that is the grace that preceeds the other works of God. Now this prevenient grace moves all humans to a neutral position so that their salvation depends on whether or not the person will cooperate with the grace God has given him or her.
Then they teach that Christ's atonment was for every human being in the same way. So there is this ... if every person is given a measure of prevenient grace and if Christ dies for every person in exactly the same way, then who really makes one man to differ from another? God has done his part. The deciding factor lies in man, not God. This is called synergism in salvation. God and man works together in bringing man to salvation.
For folks who say they hold to salvation by grace alone through faith alone in Christ alone, this poses a dilemma.