To whom, exactly, are these secrets made manifest? To God? Hardly. He's all-knowing. To others? There's no mention of others in the text. Then these secrets are made manifest to self.
The believer knows that nothing hides a secret better than his own heart. Perhaps we know more about our friends than we do ourselves, because we study them. However, we're often a complete mystery to ourselves. I’m not surprised to learn "that the heart is deceitful above all things and desperately wicked, who can know it?" (Jer. 17:9,10) Our heart keeps secrets that self can't see.
But when something approaching a true sight of self appears,
the ice of pride begins to thaw,
and like waking from slumber,
we see what was once hidden,
manifested to us by the Spirit.
Believe me, there is no surprise like the surprise of self.
It's the Spirit's good pleasure to reveal me to me, then Christ to me. Or is it the other way around? Either way, where the book of my heart, locked up and double-guarded by pride, is read aloud by the Spirit through His Word. His sanctifying work is a light that pans the walls of my dark room. When He shines upon me through the means of grace, the features of my portrait become visible.
In that light,
I see some beauty, some ugliness.
Some joy, some pain.
Some spotlessness, some filth.
It's a painfully pleasant process, this self-discovery. Because when the Spirit shines in our hearts, we can see ourselves more clearly than before, and apply to sovereign grace anew. Cleans me, wash me. I cry for the thousandth time, "God be merciful to me a sinner.” And He hears me.
So then, the secrets of the heart are not made manifest to the Spirit, but by the Spirit... to me. Paul says, "For I know nothing by myself." (1 Cor. 6)
I say, amen, Paul, amen.