There’s a principle here in the discussion about joy in Christianity and in other faiths that I think is being overlooked, although not intentionally. I will try to explain what I mean in my own poor words:
God’s love and mercy extends to every person, no matter who or where they are or what they believe. Belief is a part of our reasoning and touches our hearts and souls, but it isn’t our heart and soul. It informs us but does not tell us how to react to God’s mercies and blessings–that is for the heart and soul to do.
So, when people of other faiths react with joy to the knowledge they have of God, it’s only natural that they should. These people are what is called “people of good will.” God is touching their lives where they are because he discourages no one from devotion, even if that devotion is misinformed, for they are demonstrating their will to love him and serve him as they understand it. This applies to our separated Christian brethren, as well.
God meets us where we are, but wants us to move on. This is why staying static in any faith system, true or false, does not satisfy. But, truly, only in the Catholic Church can anyone reach the full depths of spirituality and perfection because the whole of the truth subsists within the Catholic Church. People like St. Therese of Lisieux or St. Francis of Assisi come to mind. Some people of other faiths or Christian bodies manage to reach great depths just by fulling implementing what they do understand–out of pure love for God. And that ought to be a lesson to those of us who have the fullness of faith but don’t plum the depths of it, oughtn’t it?