I came across equipollent in Smeaton on the atonement. He speaks in a footnote of two Greek words "both referring to the idea of a sacrifice, and so nearly equipollent that the one involves the other". The word, as you may guess, simply means equal in force, power, effectiveness or significance. Perhaps Smeaton has taken it from the world of logic.
Francis Bacon uses it in an essay "Only superstition is now so well advanced, that men of the first blood, are as firm as butchers by occupation; and votary resolution, is made equipollent to custom, even in matter of blood."