I think it's an interesting find, but not for the reasons that Tumulty et al. do. I think it illustrates at least three significant deficiencies in Reagan's character that many in the public don't know about (but anyone who followed his career closely knows all too well).
First, Reagan was just not that bright, and showed signs of senility in his second term. As Jonathan Chait wrote,
Lou Cannon’s biography describes President Reagan frequently misidentifying members of his own Cabinet, describing movie scenes as though they were real, changing his schedule in order to follow the advice of an astrologer, and bringing up a science-fiction movie, in which aliens cause the Soviets and Americans to come together, with such frequency that Colin Powell would joke to his staffers, “Here come the little green men again.” As Cannon concluded, “The sad, shared secret of the Reagan White House was that no one in the presidential entourage had confidence in the judgment or the capacities of the president.”
The letter confirms it. Reagan didn't know the difference between "prophesy" (the verb) and "prophecy" (the noun), and thought the correct plural was "prophesys".
Second, Reagan never let actual facts get in the way of a good story. Truth was unimportant to him. Again, anyone who's actually followed his career already knows this, but the general public doesn't -- they saw him as a genial, reliable grandfather figure. But as Stephen Greenspan wrote in Annals of Gullibility:
Many of these stories [of Reagan] were embellished or, quite typically, completely made up. One example is a story Reagan told about a football game between his high school from Dixon, Illinois, and a rival team from Mendota. In this story, the Mendota players yelled for a penalty at a crucial point in the game. The official had missed the play and asked Reagan what had happened. Reagan's sense of sports ethics required him to tell the truth, Dixon was penalized, and went on to lose the game by one touchdown. Wonderful story, except that it never happened.
This aspect of Reagan's character is also illustrated in the letter. He refers to "one hundred and twenty three specific prophesys [sic] about his [Jesus'] life all of which came true."
The claim that aspects of Jesus' life were correctly and miraculously foretold is a common one among Christian evangelicals. Oddly enough, however, the specific number of fulfilled prophecies varies widely from author to author. A google search gives "more than 300", "over 400", "hundreds", "191", "68", and many similar claims. However, most of these so-called prophecies can be dismissed right away because (a) they were not prophecies or (b) they actually referred to something other than Jesus or (c) they were extremely obscure or vague or (d) their correctness is seriously disputed.
The few that remain that might well be true because Jesus (assuming he existed) deliberately chose to take actions based on what the Old Testament said. In this case, the prophecy is correct, but not for any miraculous reason.
And of course, the value of true prophecies is negated by the prophecies that were falsified. One of the most important of Jesus' predictions -- (in Matthew 24) "Verily I say unto you, This generation shall not pass, till all these things be fulfilled." -- was falsified. None of the things Jesus claimed would happen occurred in the generation after his lifetime. The amount of ink Christians have expended trying to excuse this failed prophecy could probably fill a dozen swimming pools.
I doubt very much that Reagan investigated his 123 claims. He was not a scholar or expert in the Bible. Almost certainly he was just repeating some claim he had once heard -- this would be in line with other stories about Reagan, who had a large number of half-remembered quips and anecdotes he liked to relate, without concern for whether they were true.
Third --- and this is the most damning for me --- what the letter illustrates is the willingness of Reagan to take advantage of someone's pain and suffering to ram his religious beliefs down the throat of a dying man. Civilized people do not expect others to share their religious beliefs, and do not evangelize to vulnerable people. It is rude and it is grotesque and it is contemptible.
If, dear reader, you are a Christian and you have trouble understanding my point of view, let us try a thought experiment. Suppose you were on your deathbed, and you were very worried because, in your religion, the sins you know that you committed would likely condemn you to an afterlife of eternal damnation. Suppose I, your atheist relative, tried to console you by saying, "Look, your beliefs about Hell are all nonsense. You are not going to experience eternal damnation because THERE IS NO HELL. No heaven, either, by the way." Would you be grateful? My guess is no, but rest assured -- I would not do such a thing.
There are other aspects of Reagan's character on exhibit in his letter -- a lack of judgment, a deficiency of skepticism, and an overwhelming gullibility. But I think I've said enough: the letter is an appalling document. The fact that people celebrate it as praiseworthy indicates a fundamental sickness at the heart of modern Christian America.