The point is not that no one knows or that no one can know, or even that some are wrong. The point is what do they know? Our presuppositions must always be that the truth is knowable, insofar as God reveals it to us in Scripture.
Did Paul think he "knew" correctly?
"For the which cause I also suffer these things: nevertheless I am not ashamed: for I know whom I have believed, and am persuaded that he is able to keep that which I have committed unto him against that day." -- 2 Timothy 1:12
And was he correct?
The Jews, and Muslims, and Hindus say the same thing, yet they all 'know' a different 'truth' in their scriputres.
And was he correct?
Yes, even though I don't think his teachin was always.
All we can say is "Thank you" and "Thy will be done."
A subtle point of "for I know whom I have believed" is that he is speaking of the Person Himself, not merely what has been said about the Person or what he has observed of the Person. Paul knows Him.