Doing things in secret that you are ashamed for others to know is practical atheism. God's knowing doesn't count?Is he calling into question our salvation? Not at all. But how many of us have a secret sin, some foible that we are embarrassed about?
How often do we justify some activity by saying something like "Well, no one else knows" or "I'm not hurting anyone" or, even worse, twisting Scripture to our own ends to justify an activity ("It said not to drink to excess. 6 beers isn't excessive for me!").
Piper's point here is that yes, someone else knows. We are effectively denying God's existence by pretending He doesn't know what we're doing. It is practical atheism, as our practices and activities are denying the reality of God.
I don't know about you, but I didn't put on my steel-toed shoes this morning, and that one left a mark.