Is the cost of attending a church retreat deductable on federal income tax

[CalCountry]

See the following citation from the section titled "Conventions" in the IRS overview of Publication 17 at:
   https://www.irs.gov/publications/p17/ch24.html

" Conventions .
If a qualified organization selects you to attend a convention as its representative, you can deduct unreimbursed expenses for travel, including reasonable amounts for meals and lodging, while away from home overnight in connection with the convention.

However, see Travel , later.
  You can't deduct personal expenses for sightseeing, fishing parties, theater tickets, or nightclubs. You also can't deduct transportation, meals and lodging, and other expenses for your spouse or children.
  You can't deduct your travel expenses in attending a church convention if you go only as a member of your church rather than as a chosen representative. You can, however, deduct unreimbursed expenses that are directly connected with giving services for your church during the convention."