A first approximation to the question of what are the valid and most reliable sources for input regarding Christian spiritual formation would be that they are the canonical books of the Bible. Of course, they would need to be interpreted properly--no easy task--and perhaps they may be supplemented (less authoritatively) by the traditions of the church throughout its history in various and ways. Note: the supplemental role or “less authoritative” view of church tradition is the putative belief of Protestants, but usually restricted to traditions that are consistent with (or at least do not violate) the Bible’s overall teaching and produces good fruits.
That may be somewhat simple and obvious (to some), but it is not beyond controversy.
The reasons why this is not beyond controversy include at least the following considerations: 1) the debate over the NT canon list (affecting validity) and textual criticism (affecting reliability)--for example, its alleged “lost” gospels or the quantity of scribal errors in transmission; and closely related to that, to some degree the historical/critical method of interpretation and its critique of NT validity.
There are also 2) a postmodern view that manifests itself in the hermeneutics of suspicion (which allegedly applies to any authoritative text including the Bible), 3) the intramural disagreements that relate to plenary (contrasted with partial or no) inspiration and inerrancy views of Scripture, 4) the intramural debate among conservative systematic interpretations of Scripture, like Reformed tradition and Dispensational view of scripture and 5) controversy over the weight of authority church tradition should enjoy and what actually constitutes legitimate church traditions for modeling spiritual formation.
Besides these issues there arguably seems to be some ambiguities in the text even on important issues, and there remains beyond that, the liberal and neo-orthodox spin on interpretation models. All of this needs to be taken into consideration when one looks “under the hood” as to what constitutes valid and reliable information about Christian spiritual formation. An immature or young Christian may not need to understand all of the issues, but if you’re going to mentor and lead, the more you know about this the better. It’s part of our intellectual responsibility to believe, where we can, well supported ideas.
So the question becomes, where do we end up when we go into that much detail in order to resolve this issue? A short answer goes something like this:
The most reliable documents for mining spiritual formation data and principles are the canonical books of the NT. This seems best even though there was and has been some dispute over a few of the books that were accepted into the canon, for instance, Hebrews, Jude and as far as Luther was concerned, the book of James. On the one hand, we see no definitive reason for any of these to be outright rejected, while on the other hand there seems to be good reasons to reject what are known as the apocryphal books and the pseudopigraphal literature, including the late gnostic gospels & literature (egs. the gospel of Judas, the gospel of Thomas, etc.).
We think the most reliable literature will be that which was produced relatively early in the process of putting the life of Christ into gospel form from a likely earlier oral tradition. The authors (and in some cases their amanuenses) often had access to eyewitnesses to the life of Christ or those who had access to eyewitness accounts of the life of Jesus and the events up to and following his death and resurrection. Since some of the main issues that the Christian church faced and stood against were the influence of the Judaisers, gnosticism, and the legalism/license concerns, we therefore should be careful to evaluate their influence on any claims to authority--as being worthy of canonization.
The longer answer to those issues is much longer in that it must include justification for these choices. Since space will not allow for a full justificatory discussion here, we will refer you to some resources that we have assembled for other parts of our family of websites (Απολογία) that should be of help to you as you work to justify your thinking. See especially this link. Alternatively, see the links below and note they will open either new windows or new tabs and thus you should always be able to return to this page by closing the other windows or tabs: