It wouldn't be much of a secret motto if anyone could tell you what it means, would it?
They must be the initials of some Greek words, but which ones would be anyone's guess, and I suppose the members of Tau Kappa Epsilon will not be very forthcoming.
One of their slogans, "Faith without works is dead", is "Πίστις άνευ πράξεως, νεκρά εστίν" (pi alpha pi nu epsilon), so that's no good.
You could probably start a quiz for what these letters stand for, but make sure the prize is right.