The main problem unbelievers have with accepting the revelation of the Bible New Testament Scriptures and the Trinity - God the Father, God the Son, God the Holy Spirit - deals with exactly who Jesus was and who is He now?
John 1:1-14 reveals that the Word of God was in the beginning with God and was God, and that the Word of God became flesh. This revelation is also confirmed in Phil 2:5-11 where Paul declares Jesus existed in the "form of God" before His incarnation and birth through Mary's womb.
Those who come to a realization of who they are - a sinner, lost and separated from God - and are willing to repent as Peter declared in Acts 2:38 have no problem with the Trinity, or with the Trinity being One God.
Blessings