An interesting straw man. Have you actually heard anyone making this claim (Calvinist or otherwise)?
Again I defer to Piper:
The most agonizing problem about the assurance of salvation is not the problem of whether the objective facts of Christianity are true (God exists, Christ is God, Christ died for sinners, Christ rose from the dead, Christ saves forever all who believe, etc.). Those facts are the utterly crucial bedrock of our faith. But the really agonizing problem of assurance is whether I personally am saved by those facts.
This boils down to whether I have saving faith. What makes this agonizing — for many in the history of the church and today — is that there are people who think they have saving faith but don’t. For example, in Matthew 7:21–23, Jesus says, “Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but he who does the will of My Father who is in heaven. Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?’ And then I will declare to them, ‘I never knew you; depart from me, you who practice lawlessness.’”
-snip-
from: http://www.desiringgod.org/articles/the-agonizing-problem-of-the-assurance-of-salvation
I should also have included this par. as it get to the heart of the issue - how many of us struggle...
“So the agonizing question for some is: Do I really have saving faith? Is my faith real? Am I self-deceived? Some well-intentioned people try to lessen the problem by making faith a mere decision to affirm certain truths, like the truth: Jesus is God, and he died for my sins. Some also try to assist assurance by denying that any kind of life-change is really necessary to demonstrate the reality of faith. So they find a way to make James 2:17 mean something other than what it seems to mean: “Even so faith, if it has no works, is dead.” But these strategies to help assurance backfire. They deny some Scripture; and even the minimal faith they preserve can be agonized over and doubted by the tormented soul. They don’t solve the problem, and they lose truth. And, perhaps worst of all, they sometimes give assurance to people who should not have it. “