There is an old saying: “Those who can’t, teach.” So we get a snicker at the expense of someone who is more expert when talking than doing.
One kind of person is clever at teaching others, yet is no good whatever to himself (Sirach 37:19, NJB).
It is a great thing to be able to teach others and to be able to do it well. But at least sometimes it is far more important for the teacher to know how to put his head full of facts to its proper use. A guy who can tell you how to change a tire but can’t do it himself is going to be in trouble when he has a flat on a lonesome road, no matter what the reasons for his inability—and there are some perfectly legitimate ones.
But there seems to be more disdain conveyed by the NJB in the verse from Sirach quoted above. It is one thing to be incapable because of some defect or disability. It is quite another thing to be worthless to ourselves because of a moral flaw. If I have all the theological brains in the world but do not love God, that theology is less than worthless to me. If I am an acclaimed Sunday School teacher beloved by my students but do not conform my life to the truth out of love for God, what good does that knowledge do me? None.
Being interested in theology is a fine thing and knowing something about it is likewise great, but if I make the mistake of confusing stuff I know with actually loving God, then I may as well know nothing at all. Heck, I would be better off as a theological ignoramus if the choice is between loving God or being clever.