No man knows his Creator

"By the knowledge of God, I intend not merely a notion that there is such a Being. But also an acquaintance with whatever we ought to know concerning Him, conducing to his glory and our benefit. For we cannot with propriety say, there is any knowledge of God where there is no religion or piety. I have no reference here to that species of knowledge by which men, lost and condemned in themselves, apprehend God the Redeemer in Christ the Mediator; but only to that first and simple knowledge, to which the genuine order of nature would lead us, if Adam had retained his innocence. For though, in the present ruined state of human nature, no man will ever perceive God to be a Father, or the Author of salvation, or in any respect propitious, but as pacified by the mediation of Christ; yet it is one thing to understand, that God our Maker supports us by his power, governs us by his providence, nourishes us by his goodness, and follows us with blessings of every kind, and another  to embrace the grace of reconciliation proposed to us in Christ. Therefore, since God is first manifested, both in the structure of the world and in the general tenor of Scripture, simply as the Creator, and afterwards reveals himself in the person of Christ as a Redeemer, hence arises a two-fold knowledge of him; of which the former is first to be considered, and the other will follow in its proper place." - John Calvin, Institutes of the Christian Religion, 1.2.1

Someone might read this part of the Institutes and be unfamiliar with what Calvin is saying here. When he says 'there is no knowledge of God where there is no religion or piety', he speaks generally. He is proving the point that men are totally depraved and that there is no longer in man true knowledge of God that man believes or accepts. He is not saying that we know whether a Christian is a Christian if they perform good works or perform religious things.