Consider the following objection to the Catholic faith (this is based on something I got by email): Catholicism includes a large number of detailed and substantive doctrines that do not seem to be derivable from God's revelation as completed by around the time of death of the Apostles, even though the Catholic Church herself claims that revelation was completed by around the time of death of the Apostles.
Consider, after all, something like the doctrine that Mary was free of original sin from the first moment of her conception. This is a detailed and substantive doctrine that seems to go far beyond the information given in Scripture and what we know about the faith of the first century Church from non-Scriptural sources. The objection is an incredulous stare at the possibility that such doctrines could be derived from revelation as completed by around the time of death of the Apostles. But:
1. Twenty simple axioms of Euclidean geometry generate an infinity of detailed and substantive theorems. These theorems are such that there is no prima facie way to see that they would follow from the axioms. It can take centuries and centuries for humankind to discover that they can be derived. It should, thus, be no surprise at all that we can derive from a set S of propositions new propositions that are details and substantive, and that seem to go far beyond S. This is particularly true when S is not a list of twenty axioms, but includes about 27,570 verses of the Old Testament, about 7956 verses of the New Testament, as well as decades of Apostolic preaching which Catholics think became embedded in the tradition of the Church, particularly in her liturgy.
2. Furthermore, unlike the development of geometry which is as far as we know is typically done by the unaided human intellect, the development of Catholic doctrine is claimed to be done by the human intellect guided by Holy Spirit.
3. Moreover, the Scriptures and the Tradition of the Church not only contain particular doctrinal axioms from which we can derive further propositions, but contain ways of reasoning or rules of inference that embody an understanding of how God deals with the world. Prominent among these is typology. In the New Testament and the Church's liturgy, we learn that God works through parallels. The people of Israel pass through the sea; Christians pass through baptism. Adam sins and from his sin comes death; Christ conquers sin and from his conquering sin comes life. The New Testament (Luke 24:27) says that all of the Old Testament scriptures tell us about Christ. Thus there may be substantive ways of reasoning embodied in Scripture, liturgy and theological practice, ways of reasoning that include typological reasoning. These ways of reasoning are, plainly, more than just formal rules of logic. They are based, rather, on an understanding of God as acting in certain ways (maybe with certain motives), as producing a certain kind of deeply interconnected history.
And new insights might well come from this. Christ corresponds in an important way to Adam; but Mary in the Church's understanding corresponds in an important way to Eve. Just as Eve was created without sin, so, too, Mary was created without original sin. Now it is true that prima facie one might have tried different typological correspondences--one might, for instance, make Mary's being conceived in sin be parallel-by-contrast to Eve's being sinless (as Christ's raising us is parallel-by-contrast to Adam's bringing death on us). Working out a deep understanding of the typology here, and connecting it with many other aspects of Christian doctrine, is going to be difficult. It may take centuries, thus, for the Church to settle on a particular understanding, e.g., to see that the parallel between the new creation in Christ and the old creation in Adam does in fact call not just for Christ the new Adam to be without original sin, but Mary the new Eve as well, but of course with her freedom from the weight of original sin flowing from Christ's redemption, just as our Church's freedom from the weight of original sin does.
Conclusion: It should be no surprise if from a very large body of axioms, which includes substantive rules of inference, one could derive many doctrines that one is individually surprised by.