What is the theme of "The Doer of Good" by Oscar Wilde?
Reading this poem twice is a good idea. If you have no familiarity with the Bible, then the story may not come as easily. This is the difficulty with allusions: if someone is unfamiliar with what is being alluded to, often the message of the writing is lost.
In this case, there are two clues about the subject of the writing. First, the "voice" used sounds like a Biblical passage. For example,
'Why do you look at this woman and in such wise?'
The second clue is that the pronouns are capitalized when referring to the subject of the poem. The traveler/speaker/observer here is always referred to as "He" or "His," which is the way Christ is referred to in the Bible.
With this in mind, the two people he observes are recipients of his healing power, based on stories told in the New Testament. The first young man in the rich mansion was a leper that Christ healed. The second is a young man with lust in his eyes, and he is a blind man to whom Christ returned sight.
In both cases, Jesus asks the men why they are acting in such a way: worldly and with no thought to God, for the Bible states that the men were changed after their miracles, and not just physically. They both respond by saying something like, "Well, what did you expect?"
The theme Wilde seems to be presenting basically says that even when great things are given to us that can change us or our lives dramatically for the better, human nature is strong and difficult to overcome. It is easy to forget kindness showed us, or perhaps a humbling experience, and become what we had been before. Is this a relevant theme to our time? One might ask how many people remain connected to their fellow human beings after a terrible disaster, such as 9/11 or Katrina. How many of us are changed for a short period of time, and then return to "business as usual?"