Mystery?
1 John 5:7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
What mystery?
The mystery is whether you affirm or deny that this verse is the inspired word of God. What say you ? If you say yes, you abandon Protestant/Evangelical/Other position and admit their new translations are false and do not include the entire inspired Bible. If you say no, you deny the KJV and Textus Receptus upon which it is based, admitting the Reformation never had a complete and correct Bible all these centuries, and may not now, depending on modern scholarship rather than. Received Text. Sola Scriptura is exposed as insufficient; Catholics do not have this problem, relying on both Scripture and Apostolic Tradition for truth.
Every church I have ever been in believes that. I do not think it is a mystery at all. We are baptized in the name of the Father, Son and Holy Spirit. Where do these ideas come from?