Excellent article by a Ben Henshaw: CALVINISM AND DEUTERONOMY 29:29
He points out how Calvinists will appeal to Deut 29:29 to silence opposition, pretending their system is revelation and saying that opposing it is prying into the secret things. But he points out that in reality the Calvinist system is a claim by the Calvinists that they have access to the secret things. Their whole theory is that they know some kind of secret decrees of God and the order of those decrees! And, therefore, this verse demolishes Calvinism. A hearty Amen!