indifferentism
Concise Oxford English Dictionary © 2008 Oxford University Press:
indifferentism/ɪnˈdɪf(ə)r(ə)ntɪz(ə)m/
▶noun the belief that differences of religious belief are of no importance.
– derivatives
indifferentist noun.
indifferentist noun.
'indifferentism' also found in these Oxford entries:

