The 'unholy trinity' in Revelation is the Beast/Antichrist, False Prophet, Satan.
A mystery is something hidden. A triune understanding is revelation, not raw reason. We have sufficient revelation to know that God is one God (monotheism) and that He is a compound unity, not solitary being. We can know truth about God, but not exhaustively.
We know that there are not 3 gods (tritheism/LDS/Mormon/Hindu pagan polytheism). We know that there are not 3 mere modes or offices (oneness, modalism, Sabellianism). We know that the Holy Spirit is personal and that Jesus is God Almighty in the flesh (vs Arianism/JW/Unitarian).
We can demonstrate that there is one true God by nature/essence/uncreated spirit substance. We know that the Father is called this God, the Son is called God, the Holy Spirit is called God. We also know that the Father is not the person of the Son, the Son is not the Holy Spirit, etc.
So, within the one uncreated nature of God are 3 personal distinctions/conscious centers who are co-equal, co-eternal, co-essential. They are ontologically and relationally one yet distinct (absolutes). This is biblical, historical, orthodox Christianity, truth that is attacked by pseudo-Christian cults.
www.bible.ca/trinity