The Apostle's Creed that's said in the Catholic Church actually says “I believe in the Holy Spirit, the holy catholic church...” The word "catholic" isn't capitalized because it's supposed to mean all of Christianity, not just the Catholic Church.
The Apostles Creed was written and adopted well before the Catholic/Orthodox split and before the Protestant Reformation, so the word “catholic” does not refer to the modern Catholic Church, but to the (more or less) united Christian religion of that time, before many of today’s doctrinal differences had hardened into distinct denominations.