In summary, sola scriptura presupposes (1) the existence of the printing press, (2) the universal distribution of Bibles, (3) a cash-based economy, (4) universal literacy, (5) the universal possession of scholarly support materials, (6) the universal possession of adequate time for study, and (7) a universal education in a high level of critical thinking skills.
Needless to say, this group of conditions was not met in the crucial early centuries of the Church, was not met through the main course of Church history, and is not met even today. The non-existence of the printing press alone means sola scriptura was totally unthinkable for almost three-quarters of Christian history.
It is thus hard to think of sola scriptura as anything but the theory spawned by a group of Renaissance-era dilettantes-people who had an interest in being their own theologians, who had a classical education in critical thinking skills, who had plenty of leisure time for study, who had plenty of scholarly support materials, who had good reading skills, who had access to Bible-sellers, and most importantly, who had printed Bibles.
The average Christian today-even the average Christian in the developed world-does not fit that profile. Much less did the average Christian in the early centuries. What this means, since God does not ask a person to do what they are incapable of doing, is that God does not expect the average Christian of world history to use sola scriptura. He expects the average Christian to obtain and maintain his knowledge of theology in some other way.
But if God expects the average Christian to obtain and maintain the Christian faith without using sola scriptura, then sola scriptura is not God’s plan.
(Side note: This problem is probably why modern day Evangelicals don’t even worry so much about what the Bible teaches and would prefer to focus solely on their “personal relationship with Jesus” as a way to avoid difficult problems.)
Disclaimer – This blog post is just that: a blog post with my personal thoughts. I am not a Catholic apologist or theologian. What I say here is not official doctrine of the Catholic Church. I am still learning and am susceptible to error. Don’t take anything here as Gospel. Don’t be stupid. Do your own research and learn for yourself what the Church teaches.
Catholics: if my understanding of Catholic doctrine needs adjustment, please point out my error.