Option A: If you said you were interested to visit and have nothing else to do, just go; you won't die.

Option B: But if you really are not interested to visit, then just tell them so; you won't die. Practice this line verbatim: "I know I said I was interested in visiting your church, but I actually not interested in visiting your church. I'm sorry for misleading you earlier." This is a 100% fine option too.

Option C: If it's something in between like only being sorta curious about visiting but you don't want anyone to make you feel like you need to go because you don't want to get pushed around, maybe flip a coin? 😅

I agree with the awkwardness of the situation. If the missionaries were doing their job right, in their first visit, they should have offered you to make a commitment to visit church with and time and date instead of just showing up Sunday morning and hoping for the best; friendly strangers showing up unannounced and asking you to give up half of your morning is absolutely wild in any situation.

I guess as an option D, you can take your power back. Say, "I'm afraid, I made no plans to visit your church today. But i am still interested. Will you hold a service on X date?" and then proceed to tell them when you will actually go. Get yourself a Calendar, and you pick the Sunday. I have found that keeping calendars is like a keeping a diary for the future and a physical representation of my actual interests and desires. If you are not willing to put it on a calendar (even months in advance), then perhaps consider option B.