Audrey,
Perhaps you don't see yourself in quite the way I imagine you are seen. But, I'd suggest that coming up the drive or through the yard with them watching will be enough for them to 'know' what you've been up to for the past 1 1/2 years.

At that point I would imagine you can rest assured that the topic is 'broached.' But, their belief, their hopes and desires for your life have a long history and will not just go away because you have made your decision to transition. There's bound to be some dissonance there with all concerned. Try to remain 'in the room' and try to understand when you get there that they are being asked to 'incorporate' that which they have had no time to even know about yet. At least you've had a few years of weighing and deciding, facing you.
It will take some time for them, I would guess, whether or not they are recently confirmed Catholics. So don't set the bar too high on acceptance and embracing behavior from them. This will be a huge adjustment and it will take lotsa time before, or if, they make it. Or, at least go in expecting that it will take lots of time. If it doesn't you won't be surprised and if it does you'll be that much more fulfilled.
As for arguing religion with them, you're not supposed to. They have their beliefs and they will try to fit you into that in some way. Just listen. It's something they will work-out or not. But, more often than not, people just want to be heard. It doesn't require argument, just an ear or two. And, their arguments don't really seem to have much of a place in what you are doing, otherwise this talk and what they think would already be in play in your transition. Hear them and try to understand what they are saying, because, almost surely, they will be talking about themselves more than they will be talking about you. If you're going to question, question about what feelings this transition brings up for them, the way it impacts their hopes for you, the way they fear or embrace the way your actions will affect what they want others to see about themselves. Make the conversation about them more than about you.
All the best,
Nichole