Just curious. Can I just go in and just make like a Catholic and join the congregation as long as I have the prayers and rituals down? Are there gatekeepers that will make me prove it?
Sure. You don’t even need to know any of the prayers and rituals. As long as you’re not causing a scene, you’re welcome to participate as much or as little as you like, whether that’s as sitting quietly in a pew throughout or following along with the prayers and songs from the missalette and hymnals. Just don’t receive communion if you’re not Catholic. You can even walk through the line if you want, just don’t take a host from the minister when you get to the front. You’ll receive a blessing and go on your way.
I’ve gone to a ton of Catholic services in my life. I’m as Lutheran as they come. I don’t make the sign of the cross or kneel, but I do take the host cuz Martin Luther said it was ok 
But yeah man, everyone is welcome.
Stop you? They probably want you to do it, so you might become a Catholic by osmosis.
First you get down on your knees,
Fiddle with your rosaries,
Bow your head with great respect,
And genuflect, genuflect, genuflect!
Sorry, I couldn’t resist. Explanatory link for the kids in the audience.
Well, do you know the secret handshake?
I thought it was “Spectacles, Testicles, Wallet and Watch” 
(But I guess that was in the days of waistcoats and pocket watches)
Anecdote: When I was a kid, Catholicism was much more hardcore. There was a couple in our parish, she a Catholic, he not. He would come to Mass with her every Sunday. He didn’t kneel, he didn’t sing, he didn’t participate but he was there with his wife. EVERYBODY was cool with it. It was accepted and welcomed.
So if it was like that then, it can only be less of an issue now.
That guy participated more than many men did at the time - I’ve heard stories of men staying in until the sermon started, at which point they’d go out for a smoke, or being in the union hall while their wives and children were in church.
I went to a Catholic high school. We had a monthly mass for all students, including the half a dozen or so of us who weren’t Catholic. I had to attend, but was not required to participate beyond standing/sitting on cue. When they did communion, I just stayed in my seat until it was done.
The same is true of every other church I’ve ever attended. Services are public, and everyone is welcome. Most of the churches will have a couple of “spotters” (probably called something else) whose job seems to be seeking out new people to welcome/recruit them, introduce them around, etc.
Not all churches are welcoming
Do not accept an invitation to a Mormon wedding unless:
- You are Mormon
- You want to stand in the parking lot for 2 hours
They do check ID’s and do the sign/counter-sign routine.
Guests are not tolerated.
A Caucasian woman converted herself to Hinduism, but was challenged when she showed up at the Temple (isolated case, but may be typical). Once she established that she was a real Hindu, she was welcomed.
What’s to stop you…?
Common sense. 
Assuming I, a non-Catholic, were hellbent on receiving Communion any way, is there any way the minister could know I wasn’t eligible, or try to stop me if he did know?
The sky would cloud over and the cup in his hand turn to blood.
Don’t you have that backwards?
I’m a non-catholic who took communion as a child when I went to catholic mass once. The catholic friend I was with said it was ok. I was a devout christian back then, though non-catholic.
Nobody stopped me or said or did anything.
I grew up Catholic, and I was somewhat confused as an adult when I heard non-Catholics talking about having to have paperwork or some kind of proof of something in order to join a congregation - southern Baptist, I think. I think the only thing I’d have had to prove would be to provide a baptism certificate if I was to be married Catholic. But since we eloped, that wasn’t an issue. Plus I’ve pretty much fallen away from the whole thing.
If you get to the front of the line and address the person holding the wafers and wine as “minister,” they’ll know you aren’t Catholic.
If you have confessed and are free of your sins, and are truly set on eating Jesus, watch how others cup their hands, one on top of the other. When you get to the front, priest/deacon will say, “Body of Christ.” You say “Amen” and priest/deacon places wafer in your hands. You step to the side, put the wafer in your mouth, cross yourself (I don’t remember if there’s a genuflect in there too) and go back to your pew.
But really, as far as I recall (lapsed some 25 years) they just ask that non-Catholics not receive communion (due to the whole transubstantiation thing) and are welcome to come on in and participate in Mass. You won’t be the only one staying in your pew when Communion time comes around.
Although my atheism precludes my being catholic, I recently sat quietly and observed at a catholic funeral service. Maybe half the people did as you describe, but the other half opened their mouths and had the priest put the cookie on their tongue.
Additionally, afterwards everyone was abuzz about the “slow” guy that got in line and did the whole thing. Everyone knew he wasn’t catholic, but they didn’t know how to discretely intervene, so it just happened. People were all :eek: …
So as a kid, did I screw up? I always “confessed” my sins but I did them directly to The Lord, not to a priest in a confessional. I believed in it all and whatever. Was I “wrong” to take communion? I was in high school I believe.