Should "brother" be capitalized in a letter salutation?

John Smith, a member of my congregation, would be addressed as brother in face-to-face conversation. If I write a letter to him, should I capitalize this word? For example, as in:

Dear Brother Smith,

This supports @tchrist's analysis, but I thought I'd share the source material with you.

Here is a fairly straightforward explanation of the punctuation of the word "brother" used in religious communication from an online religion style book...

brother: A man who has taken vows in a Christian religious, particularly Catholic or Anglican, order but is not ordained. Also, a monk or friar who is in seminary preparing for priesthood is called brother if he has taken his vows. In many traditions, especially evangelical, brother is used as a generic, friendly title. Capitalize before a name but not otherwise. On first reference, generally identify the religious community, for example Franciscan Brother John Smith. On second reference, use the first name if the person is known that way, such as Brother John. Otherwise, use only the last name on second reference.

The "capitalize before a name but not otherwise" clause answers your question.

If you are using brother as a form of address, just as you might write Doctor Jones or President Obama or Deacon Williams, then yes, you would capitalize Brother John there. Compare all these:

• Dear Mister Davies,
• Dear Doctor Johns,
• Dear President Obama,
• Dear Deacon Williams,
• Dear Lady Jessica,
• Dear Pastor Johnson,
• Dear Brother Jaques,
• Dear Secretary Bird,
• Dear Uncle Tom,
• Dear Aunt Agatha,

Those are now all titles, and thus should be capitalized, particularly when used vocatively as you have here.