A mystery is something that was NOT revealed in the Old Testament but is revealed in the New Testament. The Trinity is not a mystery.
I do not accept the universal definition of the Trinity however I most certainly believe in God The Father and in His Son Jesus Christ and in The Holy Ghost I just refuse to believe they are in one body. In the garden when Christ prayed for His Apostles He prayed to His Father in Heaven, not to Himself. When asked when the 2nd coming would be He said He didn't know, that only The Father knew that. It is implausible to me that if The Father and The Son were one substance that The Son would not know anything The Father knew. If They were One then He wouldn't have to ask His Father to take away the bitter cup he was about to drink. Christ noted that He would not do His own will but that of his Father.
There are many more examples but those are enough to convince me that the Holy Catholic Church definition of The Holy Trinity is not correct.
I realize that that makes me a heretic. I don't believe that it is terribly important that I understand the exact nature of God, Jesus Christ and The Holy Ghost. No matter how wrong I am in my belief there is nobody who really knows. I am unwilling to believe that the first two hundred years of Christians were all wrong and had heritical beliefs.
Why does God say "Let US make man in our own image."? The Triune God was already present?