The Holy Trinity is the belief that God is one being in three persons: the Father, the Son, and the Holy Spirit. These three persons are distinct but fully united, sharing the same divine nature. This teaching is central to Catholicism. The Trinity is called a mystery because it is a truth about God that human reason cannot fully understand. Catholics believe it has been revealed by God, even though it goes beyond what we can logically explain. It invites faith and reflection rather than complete comprehension.