Returning to the OP …
No. First, anyone with “only head knowledge” has what demons have (according to James) and are NOT Christians.
Second point, God still reveals himself, so there are some who have what you called “first hand knowledge” … however, this is not as valuable as you seem to think since FALSE first hand knowledge is just as real to the individual as TRUEE first hand knowledge (remember the warnings about testing spirits and masquerading as spirits of light).
Third point, the apostolic letter writers contain an important piece of information that will allow any Christian to “self examine” without need of special revelation: we are changed. The transformation wrought by TRUE SALVATION is the evidence of the “spiritual truths”. That which does not prevent God transforming your heart is not an “essential” and any false “essential” will prevent a genuine salvation.