There are plenty of ways to interact ecumenically with Protestant brothers and sisters. E.g., I play piano at a Protestant church. I enjoy staying for their coffee and donuts and chatting with everyone. I love praying with the congregation, and of course, I love leading them in the hymns (so many good hymns, and they love to sing!).
But what it all comes down to is obedience. The Catholic Church tells me in writing (see CCC quote above) and in personal teaching (we heard this in our RCIA classes, and we hear it at least a few times a year from the priest’s homily, and we also hear it in our parish Bible studies) that we are not permitted to receive Protestant Communion.
Here’s how I look at it–when I am playing and interacting with a Protestant church, and occasionally answering questions about Catholicism, the people see that a Catholic really IS a Christian! (Note–there are many Evangelical Protestants who believe that Catholics are not Christians because they believe that Catholicism is a cult). I am a witness to them about the joy and truth of Catholicism, especially when I am able to quote the Bible and describe a recent Catholic homily or article (sometimes on CA!).
And when they see that I do not receive Communion, and when they ask, I tell them that in spite of the bond of belief in the Lord Jesus and the joy of worshiping together and our bond of Christian love, Catholics are not permitted to receive non-Catholic Communion because it is confusing to Protestants and gives the impression that we accept all their beliefs., They not only understand, but they say, “That makes sense.”
Obedience with understanding impresses Evangelical Protestants.
I feel that I do more good to Protestants by obeying my Catholic Church and abstaining than I do by disobeying the Church and receiving their communion. Just as in the Catholic Church, we can occasionally (for a valid reason, such as not getting to confession for a mortal sin) abstain from receiving Holy Communion and instead, have a spiritual communion, we can do the same thing in the Protestant church–enjoy spiritual communion and fellowship with Protestant brothers and sisters.
We need to trust God–if He has directed His Church, the Church that He founded, to do or not do something, we should obey, even if we disagree or have doubts about the instructions. Look at all the trouble we have when Catholics question what the rubrics of the Mass should be, and make up their own, or refuse to follow the Church-prescribed rubrics because of a personal conviction that “the Church is wrong.” (This happens a lot with the Sign of Peace–sigh).
How do we expect non-Christians and non-Catholic Christians to join the Catholic Church and have unity once again (John 17) when we do things our way instead of God’s way?