So you can academically believe in God, yet you don't really know you really do because the devil can emulate it. And all that wonderful affection and delight in God's glory, that too can be simulated by the devil? Odd, we can infallibly discern sin and hatred in our hearts, but we can't discern virtue to the same level of certainty??? If we follow this through, we can't even know if our understanding is genuinely of God or of the devil...
Your second paragraph, speratus, does not resolve anything. If we cannot KNOW we believe, then how can we appropriate to ourselves with certainty any promise of forgiveness?
I have read the Puritans, Covenanters and Reformers on this subject, and the more I read, the more muddled the issue becomes. Joel Beeke's "Quest For Full Assurance" deals with many of the multifarious views Calvinists have had on this issue. It seems to me that there is great confusion in terminologies, even among those who tried to treat the subject in a very methodical way. Pedantic semantics don't help; appeals to mystic impressions or "whispers of faith" help even less. People like Donald Cargill claiming to have heard voices assuring them of their salvation adds to the confusion.
I stand by the simple premise that if a person knows he loves God and desires His glory, even if only a little, then that person may be assured that he has agape love and has therefore repented and has an interest in Christ. If the love we see is selfless, it IS agape love, worked by the Holy Ghost.
