By “prophecy” here is plainly meant the same prophetic word designated in the preceding verse. Yet it seems to have a narrower meaning here.
It does not mean the prophetic book, or the whole collection of books of the Old Testament Sacred Scripture as the entire word of God.
It means a prophecy, or one or another prophetic saying in the book. For this reason the Apostle here says “prophecy,” whereas in the preceding verse he said “prophetic word.”
This detail may have significance for clarifying the concept of the resolving, or interpretation, of prophecy in this verse.
“Prophecy of the book,” or prophecy contained in a book. By this word the Apostle apparently meant to designate true prophecy, in contrast to the divinatory utterances of false prophecy.
Above he called such utterances cunningly devised myths (2 Pet 1:16).
“It cannot be resolved by oneself.” “It does not come by its own declaration.” To resolve means to explain, to interpret, and more particularly to solve something enigmatic (Mk 4:34).
“By its own declaration.” By whose declaration, or resolution, or solution? Either by the resolution of the enigmatic saying itself, that is, the prophecy of the book.
Or by the resolution of the one to whom the task of resolving is proposed, the man himself who resolves or interprets.
In the first case, the explanation, or declaration, has in view the very content of the prophecy interpreted. In the second, it has in view the interpretation of the interpreter.
In the first case the expression means that what is interpreted cannot be resolved or interpreted by itself. In the latter, it means that the interpreter cannot resolve or interpret what is interpreted by himself.
In both cases, resolving or interpreting prophecy requires a means, or mediator.
This mediator is either God Himself, who inspires the prophet, or the inspired prophet himself.
The meaning of the saying will be this. No prophecy in Scripture can be resolved by anyone on his own, except by God Himself or by the prophet.
If God or the prophet himself does not interpret the prophecy, no one will interpret it. It will remain uninterpreted and unresolved.
God Himself interprets prophecy either directly through the prophet himself, through other prophets, or through the fulfillment of the prophecy in events.
These events can be understood and interpreted by all people who attend to the ways of God’s providence in history.
Regarding the latter, that is, all interpreters of the prophetic word, or of prophecy in general, one must say that for them the means, or mediator, in interpretation can only be the prophetic word itself in its broad sense.
That is, it is the prophetic revelation of God in general, in the word or in Sacred Scripture.
Generally speaking, Sacred Scripture must be interpreted by Sacred Scripture itself, of course under the guidance of the Church of the Old and New Testaments.
The prophetic word in general, like the apostolic word, cannot be handed over to the interpreter’s whim or to the explanation of every reader.
Otherwise there will be no firmness or certainty in opinion and interpretation concerning the most sure word.
The proof of the truth of this position is the Apostle’s further saying about the prophetic word, namely, that the beginning and root of prophecy are in inspiration from the Holy Spirit (v. 21).
Therefore, just as the origin of the prophetic word is from the Holy Spirit, so its interpretation must be under the guidance of the Holy Spirit.
That is, it must be under the guidance of the very word of God, or Sacred Scripture.
One may even say that the saying under consideration, that no prophecy in Scripture can be resolved by oneself, is in essence not so much about resolving or interpreting prophecy.
It is rather about the origin of prophecy, as is clearly seen from verse 21.
Thus the force of the verse being explained could be expressed as follows. In its origin, prophecy in Scripture is not subject to explanation or resolution by itself.
For prophecy was never uttered by the will of man (v. 21). It was involuntary, inspired by God. Therefore by the Spirit of God it must also be resolved or interpreted.