I too, am an unabashed atheist. My advice to you: Be yourself. You’ll be happier in the long run.
Think: Is your boyfriend a devout Catholic? If he is, you might have to face that things may not work out as you hope. Religion poisons everything, don’t let other people’s beliefs impede your happiness.
If he’s not, then you should still proceed with caution, particularly around his relatives, expect dissent, but don’t tolerate insult.