What about Celtic Religions and the early religions of many countries.
As far as I can tell, there was no effort made to proselytize these religions. When Caesar wrote his Memoirs, he simply referred to the gods of different people he encountered in an offhand way, saying they were local versions of Roman gods he knew. Various religious leaders might have had their own ideas about spreading the word, but if so, they did it strictly on a local basis. I can't say I've encountered any indication of people trying to spread the worship of Isis, Osiris, Athene, Zeus Melagrius, etc.
In fact, when the Christians first showed up in Rome, contemporary reports (Livy, I think) seem to be, well, astonished. The Romans were amazed that these foreigners felt they had to riot and destroy the temples of other gods. It seems to have genuinely caught the locals out, as though they never encountered such behavior before.
That's not to say the gods of early historic Europe were necessarily all alike, but under different names. I mean, a solar Roman deity such as Mithras could become very popular in ancient Britain, given the movement of Roman soldiers (who worshipped him--Plutarch, Life of Pompey), but a solar deity in ancient Mesopotamia (whose name escapes me at the moment--I hope he doesn't mind) could be a largely negative force, connected with the destruction caused by the desert sun.
But by and large, it looks like military conquest was free of religious triumphalism, and people were allowed to worship their own mysteries that whispered of unique roads leading to an oddly similar goal. Extremely civilized, yes?
