Simply be who you are. No need for grand announcements that would most likely upset everyone involved. If you are still living under their roof, depending on their support, then sorry, you have do as they dictate if they tell you that you have to join in religious activities despite making it clear you do not wish to.
There is also the matter of respect. My aunt is a holy roller, when I enter her house and sit at her table, I will bow my head while they pray. I won’t pray, or at least I won’t pray to the God they wish me too, but I will still show them the respect I expect them to show me, even though they don’t and try to belittle my ideas on the matter of faith.
@ducky_dni made a good point. If they are xtians and true ones (although I find them exceedingly rare), they may not like your choice, but they will still love you. Something about loving the supposed sinner and but hating the sin.