In More than a Carpenter, McDowell makes four key points based on passages from the synoptic Gospels in support of his Trilemma argument. I am now focusing on the first point:
...Jesus claimed to be able to forgive sins. (MTC, p.18)
I have considered and rejected one argument that McDowell presents relating to this point:
1. Jesus forgave sins.
Therefore,
2. Jesus can forgive sins.
3. Only God can forgive sins.
Therefore,
4. Jesus is God.
This will not work as an apologetic argument, because the two main premises (1) and (3) are controversial theological claims that are not subject to empirical or historical evaluation.
I have suggested a modification of this argument to try to get around the objection concerning the use of controversial theological claims as premises in an apologetic argument:
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.
Premises (2a) and (3a) are not controversial theological claims, and they are both subject to empirical and historical evaluation. What Jesus believed or did not believe is an empirical question. It might be difficult to arrive at a firm conclusion on this matter, but there is relevant historical data that can be examined and that might either support or disconfirm specific claims about what Jesus taught and/or believed.
It should be noted that the inference from (2a) and (3a) to (4a) is not a valid deductive inference. It is logically possible for (2a) and (3a) to be true, and yet for (4a) to be false. However, the combination of (2a) and (3a) do constitute a good reason for accepting the conclusion (4a). They make the conclusion probably true.
It is a very simple and obvious inference to go from "I can forgive sins" and "Only God can forgive sins" to "I am God". So, if we assume that Jesus held the first two beliefs, it would be reasonable to conclude that he put two and two together and drew the obvious inference that he was God. So, although this modified argument is not a valid deductive argument, the reasoning is good enough to make the conclusion probable, given the truth of the premises.
The passage McDowell cites from the Gospel of Mark certainly provides some evidence in support of premise (2a). How strong this evidence is will require both a general evaluation of the historical reliability of the Gospel of Mark and also a more specific historical evaluation of the passage in question.
Premise (3a), however, is not supported by appropriate evidence, and it is not at all clear whether this claim is true. McDowell briefly argues for the belief that "Only God could forgive sins":
By Jewish law this was something only God could do; Isaiah 43:25 restricts this perogative to God alone. (MTC, p. 18)
But here McDowell is trying to defend a controversial theological belief. What is at issue is not whether this belief is true, but whether Jesus had this belief. The latter issue is one that is subject to empirical and historical investigation. Quotations from Isaiah or from the Old Testament do not provide strong evidence for what Jesus believed, particularly on such a fine point of theology. Instead, McDowell needs to present some Gospel evidence about the words and teachings of Jesus. That is how one supports a claim about what the historical Jesus did or did not believe.
Perhaps McDowell was thinking along these lines:
5. Jesus believed each and every theological claim taught by the Old Testament.
6. The Old Testament teaches the theological claim that "Only God could forgive sins".
Therefore,
3a. Jesus believed that only God could forgive sins.
Although premise (6) could be challenged, the main problem here is premise (5). Why should we assume that Jesus would believe each and every theological claim taught by the OT?
McDowell, and other Evangelical Christians believe that Jesus is God, and that the OT was inspired by God, so it would make sense from that point of view to assume that Jesus was familiar with each and every theological claim made by the OT. But such clearly Christian assumptions beg the question in the context of an apologetic argument, which is an attempt to rationally persuade a skeptic of some basic Christian belief. Such assumptions are legitimate for in-house discussions between Christian believers and theologians, but they have no place in an apologetic argument.