Translation:His father was very religious, I think.
It is more a question of what is normal usage in English. French speakers tend to talk about someone being a believer, English people tend to talk about someone being religious, or not, as the case may be. Just imagine you were talking about a mutual aquaintance, explaining why she wasn't allowed to do something or other when she was young...
There is a big difference between being religious and believing. Religiosity is merely routinely going through the motions; you don't have to believe to do that. And, believers do not have to be religious. I would much rather be a believer that to be simply religious. It's the heart that counts, not simply showing up in church.