I knew practically nothing about the Catholic Church before I attended my first Mass. Only with me, it was a co-worker who did the inviting and I said yes before I realized what had happened! To make a long story short, that was the catalyst that led me to where I am now - in RCIA to enter into full communion with the Catholic Church at Easter.
Last week at RCIA, we were talking about liturgy, its importance, significance, etc. The priest said something that really rang true for me - you can’t begin to explain liturgy (including Mass) to someone until that person experiences it. With me, my friend told me very little - she reassured me that it was okay if I didn’t follow everything and what to do when everyone proceeds up to take communion. We also got there early and she showed me how to use the missal in the pew to follow along (not only did it help with being able to participate more fully, it gave me something to hold onto - most Protestants are used to having to flip through their Bibles to follow a pastor through a sermon). At the time, I didn’t understand what was taking place, but I could sense it was “big.” By the grace of God, the experience sparked my hunger to learn more and understand. I started asking questions of my friend, who was encouraging and patient in answering them, and I started doing my own research.
There is SOOOO much that an evangelical might not understand or wonder about, from the meaning and use of holy water, the statutes, candles, alter, etc., the gestures, and the liturgy itself, without even getting into the Real Presence of Our Lord and Savior in the Holy Eucharist, it would be difficult and potentially overwhelming to them if we tried to cover all the details before they had a chance to experience them firsthand. Initially, I would recommend giving them enough information and reassurance so they don’t feel too self-concious and then be prepared to answer many questions.