For some liberals, it absolutely is. Religiosity is a biological inclination; you have it or you don’t. Someone who is brought up faithless but is religiously inclined is going to find some other normative system in which to believe in unconditionally —their parents’ politics being readily to hand. The parents who would have brought up an irreligious child in the 1960s or 1970s are very likely to have been liberal ... so you get your preponderance of liberalism-as-religion folks. Irreligious families now are a lot more common and politically diverse so it will be interesting to see how this dynamic changes.
You know, I think this is true.