Religious group rejecting Methuselah's creed and age, initially (1,1,1,1)

'religious group' is the definition.
(Young Men's Christian Association)

'rejecting methuselah's creed and age initially' is the wordplay.
'rejecting' says the letters should be written in reverse.
'methuselah's' becomes 'my' (where the crossword setter is 'Methuselah').
'creed' becomes 'c' (this might be a standard abbreviation of which I'm unaware).
'and' says to put letters next to each other.
'initially' says to take the initial letters.
The first letter of 'age' is 'a'.
'my' written backwards gives 'ym'.
'ym'+'c'+'a'='Y M C A'