Considering the fact that you can only purchase an Affiliation to 3D, and Visionary Faith equals your Cult Affiliation + 3, is the maximum you can reach B6 with Visionary Faith? Or am I missing something?
B6 is only the maximum starting Faith for Visionaries
Cult Affiliation + 3 is just how you calculate the starting value for Visionary Faith during character burning. A Roden’s Visionary Faith works the same as other varieties of Faith and advances during play, like other Emotional Attributes do. (Faith in particular only counts successful tests.) The starting exponent (and shade!) of any kind of Faith are only the most you can get during character burning to start play with. This formula is provided because any Emotional Attribute needs some way to be determined at start, since they aren’t covered by the rules for determining the standard Attributes.
(If you don’t want a giant cult, consider not buying its Affiliation up to B3. Aside from the high cost in RPs: mo’ cult, mo’ problems. Since you can advance a Visionary Faith B4 or B5 during play, a B1 or B2 cult is also fine. Get that big cult only if that’s what you actually want for the character. Then again, having a big cult just for its power and its bump to Resources is itself a worthwhile story of personal corruption…)
In general, Visionary Faith follows all the rules for Faith on BWG(R) pages 522–7 that aren’t replaced by its own rules. See also page 206 for some of the explanation of how human Faith attributes are started. As all other Faith-type attributes are modifications of the rules for human Faith, this is useful context. Faith variants are all written assuming the players and group are familiar with the normal Faith rules.
The “Religion” chapter in the Codex, pages 270–8, would be good to read too, as this section has lots relevant to cults. It contains another example of a Faith variant (Faith In Dead Gods) to compare and contrast with Faith and Visionary Faith, too.
Also notice that human Faith has a starting max of B6 as well: B3 base plus 1 for each of the three questions answered “correctly” on page 206, so Roden Visionary Faith starting with a max of B6 is on par.
And as always when playing Roden, read everything on Codex pages 451-9! This goes double for Roden cult leaders.