Let’s be very careful here. The Church is not telling us that rock 'n roll is sinful. What the Church is telling us is that music used in liturgy has to be specifically liturgical. In other words, the music (including the lyrics) has to be the special. We should be able to hear a piece of music and say, “Hey that’s church music.” I just made up that sentence; but you get my idea. I hope
In other words, the music that we use for entertainment is for that purpose. The music that we use to sing children to sleep, is for that purpose and so forth. Just as we have genres, instruments and lyrics that are easily identified with certain situations: entertainment, romance, movies, children and so forth, we must make sure that the music that we use for mass is easily identifiable with liturgy and not easily confused with something else.
The reason is simple. Liturgy is a sacred space in our day or our week, if we only attend on Sundays. The bes way to understand liturgy is to undersand the language of the Eastern Catholics and Eastern Orthodox. They refer to it as Divine Liturgy. The word Divine is key. What we say, do and sing at mass has to connect our heart, mind, and senses with the Divine, because the Divine makes himself present through the Word and the Eucharist for us. We have to rise out of our usual and go out to meet him.
To rise out of our usual, means that everything that we do in liturgy has to be different from our usual.
I hope this helps and does not offend.
Fraternally,
Br. JR, OSF