W. Robert Anfill, a Catholic writer, asks how kingdom exclusion is not simply a Protestantized form of purgatory. In an article for Catholic.com, he writes,
Many Fundamentalist Protestants interpret the Bible in accordance with the so-called “Dispensationalism” popularized by the Scofield Reference Bible… Dispensationalists go beyond even the classic Lutherans and Calvinists in their insistence on the “eternal security” of the believer, often insisting that even utterly fruitless and dead faith (cf. James 2) is saving faith! For this they are condemned by Calvinists for fostering a lawless or antinomian mentality. The dispensationalists counter this charge by affirming that, though no believers will be damned, some will be more highly rewarded for their good works and service than others; and fruitless or “carnal” believers will, in the Day of Judgment, even feel a temporary deprivation of the fullness of joy and glory.
See article at Catholic.com: Purgatory in all but name.
J.D. Faust writes in The Rod, Will God Spare It? that Kingdom Exclusion is not a type of “Baptist” purgatory, but he does little to explain the distinction. He does not say what purgatory is, nor does he explain how KE is not simply a reworking of the Catholic idea of purification and purgation.
Certainly, KE and purgatory are not the same, but they are not very different either. Both propose that carnal Christians will be purified/punished before attaining full salvation. Faust believes this will be realized in the millennial kingdom; the Catholic Church in purgatory. The nature of that experience and its duration varies, but beyond that, the ideas are the same.
The modern Protestant form of purgatory appears to originate with Watchman Nee (I can find no earlier proponent of the teaching). He held that carnal Christians will be purified in the lake of fire (cf. Rev. 20 — though that text does not mention carnal Christians, but unbelievers). Arlen Chitwood holds a similar view, except he sees exclusion as a form of punishment, not purification. Faust holds that exclusion is punitive, but that carnal Christians will be beaten with a celestial rod, not cast into the lake of fire.
(For an analysis of Nee’s view, visit http://www.bcbsr.com/topics/lc.html#purgatory.)
Strangely, exclusionists like Faust, Chitwood and Nee would have a stronger position if they adopted the Catholic stance, for, as it stands, they possess the idea, but no real evidence for it. KE/Purgatory is not represented in the canonical texts, but it is attested in some of the writings of the church fathers. Without those witnesses, KE rings hollow.