Here is my first attempt at revising an argument given by McDowell in support of the premise that Jesus claimed to be God:
2a. Jesus believed that he could forgive sins.
3a. Jesus believed that only God could forgive sins.
Therefore,
4a. Jesus believed that he was God.
I have objected that McDowell failed to provide relevant evidence to support premise (3a), and it is unclear whether this premise is true.
Another problem is that this argument is about what Jesus believed, but the premise of the Trilemma that needs support is about what Jesus claimed. So, I will make another attempt at reconstructing McDowell's reasoning.
Since McDowell does specifically talk about Jesus claiming to be able to forgive sins, that would probably be a better initial premise:
2b. Jesus claimed that he could forgive sins.
The conclusion, in order to provide relevant support to the Trilemma, must also be about what Jesus claimed:
4b. Jesus claimed to be God.
The question is, how do we bridge the logical gap between (2b) and (4b)?
I think a couple of generic premises about the context of (2b) will work, at least as logical place holders:
2b. Jesus claimed that he could forgive sins.
5. Jesus made the claim that he could forgive sins, and he did so in a context of type X.
6. The claim that one can forgive sins, when made in a context of type X, carries the implication that one is God.
Therefore,
7. Jesus asserted a claim that in the particular context carried the implication that Jesus was God.
Therefore,
4b. Jesus claimed to be God.
Since premise (5) presupposes the truth of premise (2b), we can drop (2b) without impacting the logic of the argument.
This reconstruction of McDowell's thinking appears to avoid the problems of the previous arguments. Premise (5) is an empirical and historical claim (at least it will be if we can properly fill in the blank to define "a context of type X").
Premise (6) might not be an empirical claim, but it is a claim about the meaning or use of the expression "I have the power to forgive sins" (and similar expressions). This is a conceptual claim and there are commonly accepted ways of evaluating such claims--no need to appeal to divine revelation or mystical experience to settle this question. Finally, the conclusion is one of the premises of the Trilemma, so there is no question about the relevance of this argument to the Trilemma.
The trick here is to define what is meant by "a context of type X" in such a way that the conceptual claim in (6) is true or plausible, while at the same time there is adequate historical evidence available to support (5), given the clarification.
The possibilities and permutations that one could think up to fill in the blank here are too numerous to ever allow for a complete refutation of the general form of the argument above. So, I will have to content myself with looking at just a few possibilities for defining "a context of type X".
One possibility comes to mind. What if the people who were listening to Jesus when he made the claim to be able to forgive sins were people who believed that only God could forgive sins? In such a context, making the claim to be able to forgive sins might carry additional significance, especially if we contrast this context with one in which the people listening held the contrary belief that all (or many) human beings had the power to forgive sins.
Definition 1: Some person P makes a claim C in a context of type X if and only if P makes a claim C to a group of people who believe that only God can forgive sins.
Does the above argument work if we fill in the blank using Definition 1 (hereafter: D1)?
On (D1) claim (6) is subject to the following counterexample:
Jesus did not believe that only God can forgive sins, and he made it plain to other people around him that he did not believe this.
In this situation, even though people around him might take the content of Jesus' claim to logically imply that Jesus was God (i.e. they would believe that "If Jesus truly can forgive sins, then Jesus must be God."), those people would not attribute such a belief or inference to Jesus, and thus they would not interpret Jesus as intending to assert or imply that he was God. They would understand that from Jesus' point of view, the ability to forgive sins is no big deal, that from Jesus' point of view this does not carry the implication that Jesus was divine.
Another problem, a problem that might well plague all attempts to fill in the blank, is that we don't have solid historical evidence about the beliefs of the people that were gathered around Jesus at the time he (allegedly) made the claim to be able to forgive sins. Thus, once we use (D1) to clarify the empirical premise (5), we would not be able to determine whether (5) was true or not. It might be true, but it might not be.
Certainly some of the people around Jesus believed that only God could forgive sins, at least according to the Gospel account. But did all of the people around Jesus believe this? If only a few people believed this, and others disbelieved it, then how could the claim be reasonably interpreted to carry the implication that Jesus was God?
To be continued...