Providing some methodological clarity and specificity to the topic of intertextuality in biblical studies, is the careful work of Christopher Beecham, which I draw from in this post. By specifying and clarifying his criteria, something many engaged in intertextuality have failed to do, Beecham brings guidance to an at times unclear conversation.
Items that are Essential to Identifying Something as an Allusion.
1. First, an allusion is an intentional, conscious attempt by an author to point a reader back to a prior text.
2. The second item that is essential to allusion is that an allusion has “in each instance, a single identifiable source.”
3. Third, an allusion must adequately stand out in order to be perceived by the audience. This presupposes that the author and reader share a common language and tradition. For an allusion to be successful, the prior text must be “. . . part of the portable library shared by the author and his ideal audience.” If the work is unfamiliar to the reader, the allusion will race past the ear like an arrow that missed its target.
4. The final item essential to allusion is that an author employing it expects that the audience will remember the original sense of the previous text and link the appropriate components that the new context requires in order to be most fully understood.
Items that are essential for identifying an Echo
1. First, unlike allusion, an echo may be either a conscious or unconscious act. Echoes are faint enough that often it is impossible to gauge whether its appearance in the text was consciously or unconsciously performed by the author.
2. Every echo derives from one specific text, event, tradition, person, or thing (whether animate or inanimate, concrete or abstract). If the echo is a textual or literary echo, it stems from a text that the author has read (or heard) at some point in the past.
3. Third, unlike allusion, by echo the author does not intend to point the audience to the precursor. Intention implies a conscious activity, and echo is often but not always a conscious act. Echo is a linking of texts accomplished without the aim to render a communication for public consumption… Echoes surface in a text largely because the author’s mind is saturated with the source text.
4. Unlike allusion, an echo is not dependent upon the original sense of the precursor to be understood. The meaning in the new context is not tied to the previous context; that is, the audience does not need to “recognize, remember, realize, and connect” the two texts to grasp the author’s intended public communication in the new context. The original context may or may not have been taken into consideration.
The strongest, most explicit mode of reference is quotation. The citation of the former reference is verbatim or nearly so, and is long enough to be recognized as such. An allusion, while still overt by definition, is less explicit, being more “fragmentary or periphrastic.”
Quotations may be further divided up into two categories: formal and informal. A formal quotation is a quotation that is accompanied by a quotation formula, which serves as a clear marker to the reader that what follows (or immediately precedes) is a citation from a previous source… An informal quotation, on the other hand, is a quotation that lacks a quotation formula. An informal quotation is just as much a quotation as a formal one; it merely wants for an explicit introductory marker.
This is from Christopher A. Beetham. Echoes of Scripture in the Letter of Paul to the Colossians. Biblical Interpretation Series 96. (Leiden: Brill, 2008), 16-20.