In explaining how many of the medieval Roman errors came into being, Richard Field relies on men like Jean Gerson and William of Ockam. He also displays a strong grasp of the patristics in his own right.
Regarding the subject of purgatory, Field states that the idea that the Pope, or anyone else for that matter, could dispense extra merit to advance the soul from purgatory to heaven was a totally unheard of notion. Tied as it is to the larger Roman soteriological system, Field is confident in his assertions of Rome’s fraudulent claims to antiquity.
But regarding the actual history of purgatory, Field admits the story is more complicated. This is where many evangelicals are easily confused, by the way. They have certain assumptions about “the early church,” and in the event that something appears in the early church, in “seed form” as Newman would say, they assume that the later development is thus vindicated. In this instance of purgatory, however, this cannot be the case. Field explains:
But if we speak of a declination from the sincerity of the Christian faith, it is certain it began long ago, even in the first ages of the Church. Of this sort was the error that the souls of the just are in some part of hell till the last day, as Tertullian (De Anime, c. 55) , Irenaeus (Contra Haereses v.31), and sundry other of the ancient did imagine (Sixtus Senens. Biblioth. lib. vi. annot. 345); and that they see not God nor enjoy heaven’s happiness, till the general resurrection, which was the opinion of many of the fathers.
That all catholic Christians, how wickedly soever they live, yet holding the foundation of true Christian profession, shall in the end, after great torments endured in the world to come, be saved “as it were by fire.” This was the error of sundry of the ancient, who durst not say as Origen, that the angels that fell shall in the end be restored: nor, as some other, mollifying the hardness of Origen’s opinion, that all men, whether Christians or infidels: nor, as a third sort, that all Christians, how damnably soever erring in matter of faith, shall in the end be saved: but thought it most reasonable, that all right believing Christians should find mercy, whatsoever their wickedness were (Hieron. in comment. in Esaiae lxvi.; Aug. de civitate Dei, li. 21, cap. 24, 25, 26, 27)
~Of the Church, Bk. III Chpt. 9 p 176
Purgatory used to be Hell, from which men were eventually redeemed. Though some teachers could maintain universalism (Gregory of Nyssa comes to mind), many could not, and the doctrine “developed” fairly drastically.
This sort of reading of Church history is not of much comfort to the recent ex-fundamentalist. In rejecting the “great apostasy” theory of their forefathers, they earnestly hoped to find some sort of respectable pristine “early church” upon which they could base their theology. What they will actually find is much messier.
Of course, in rudiments and genealogy, we all come from the “early church.” However, when it comes to systematic theology, there is an awful lot of water under everyone’s bridge.
You can read all of Field’s Of the Church here.