I'm reading over again a bunch of stuff on the North African Fathers for my history of Christian thought class at the Institute of Lutheran Theology- and it occurred to me how good it was that people in early Christianity eventually changed their minds about the idea that you couldn't be forgiven post-baptismal sins unless you died as a martyr. Beyond that fact that this stands in contradiction of the Scriptures and the biblical gospel, it would have been an unsustainable in the post-Constantinian Church. No one was dying as a martyr anymore and so, how would people ever get forgiven?
But I was also thinking of some sort of sci fi scenario (or alternative history- think Larry Turtledove!) where in modern American Christianity this was still the expectation. Would we have people buying tickets in their old age to go to countries hostile to Christianity just so they could figure out a way to get martyred? Older southern Baptist ladies spending their last retirement funds to go to Pakistan perhaps? That would be deeply odd.
The question still remains: How did they come up with this bizarre theology? Perhaps that isolated passage in Hebrews that sort of sounds like you can't sin after baptism? Personally, I doubt that that can entirely account for it.
For an alternative theory, here's what I've been thinking. I can sort of see how they came up with this idea based on how one views the function of baptism. If baptism is a definitive break with the power of sin (as doubtless Paul says it is in Romans 6!) how does one conceptualize this while allowing for post-baptismal repentance? It's hard. Why? Because if baptism forgives sins and regenerates, and subsequent repentance does the same thing, how are these things functionally different- and beyond that how is baptism special? It just seems one instance of forgiveness and sanctification among others. Ergo, if it's special and definitive, why not conclude that it's the only instance of forgiveness and regeneration! Again, all the language in the NT about baptism being the definitive break with sin could easily be seen as supporting this.
One can therefore see this logic worked out or modified in different traditions depending on what implications they historically drew from this. The Roman Catholics and Evangelicals deal with this problem differently. For RC's baptism is a definitive break with sin and this can't be repeated. RC's then say that penance repairs the complete reality of baptism- this was the way they ultimately came up with of dealing with the fact that people did sin after baptism in the ancient Church. Of course, this then created the problem of the fact that it appeared that the work of the penitent was supplementing the work of Christ and also that people weren't getting all their penance done on earth before they died (hence purgatory!). Subsequently the tried to solve the former difficulty by distinguishing between temporal and eternal punishment, and by saying that penance was an entry into the sufferings of Christ rather than supplementing them.
Likewise, Evangelicals and most other low-church Protestants see baptism as symbolic of initiation into the Church. If it were a break with sin, it would only be one instance among many. Consequently, it must not be a real break with sin, but only a symbolic entry into the community and perhaps (if you're Baptist) the public pledge that you're really going to try to live the Christian life now. The idea of a public pledge of really, really trying to work hard to avoid sin is how they reconcile the language of the NT about the break with sin with the fact that they actually don't believe that baptism does anything.
In light of this, the Lutheran understanding of baptism and repentance takes on new significance. Baptism is the definitive break with sin. It is the end and there is no moving beyond it- moving beyond it is moving away from it and is by definition sin. Hence, repentance does not supplement baptism, but is a return to baptism. This is all rooted in Christology. Since baptism is dying and rising with Christ, and Christ is present in all of human history according to both natures (he is confined to neither time nor space- this being due to the genus majestaticum), this presence allows for our continual return to the ever present reality of his death and resurrection through divine word of promise. Other Christian groups who deny the genus majestaticum and therefore Christ's omnipresence according to both nature, don't have that possibility. For this reason, baptism remains as something that happened in the past. It can only be repaired in the present through some supplemental act (penance) or naturally degenerates into merely symbolic act of having joining the Church. The only other alternative would be to return to the position of the ancient Church and deny post-baptismal sin.