Abdullah,
The plurality of persons in the "One God" of the old Testament has been explained to Muslims and non-Messianic Jews over and over again, yet it is rejected over and over again, mainly because you do not accept or believe in the life, death on a cross, and resurrection of the Lord Jesus Christ who now sits at the right hand of God the Father (Acts 2:33).
Jesus declared this in John 17:5,
"Now, Father, glorify Me together with Yourself, with the glory which I had with You before the world was."
The mystery of the Trinity may not have been clearly understood to old Testament Believers, but with the coming of Jesus, there is no question as to what Jesus said or meant in John 17:5. Jesus declared He shared glory with God the Father before the world was, and would share this glory again with God the Father after going to the cross. Pentecost and the continuing 2000 years of Pentecost in the lives of Believers is living proof that Jesus is Lord, and will come again as King of kings and Lord of lords (Rev 19:16)!
1 Tim 3:16 declares,
By common confession, great is the mystery of godliness: He who was revealed in the flesh, Was vindicated in the Spirit, Seen by angels, Proclaimed among the nations, Believed on in the world, Taken up in glory.
No prophet who ever lived declared he shared glory with God before the world was. Jesus did, and Pentecost proves He was telling the truth.
"For God so loved the world, that He gave His only begotten (Acts 13:33) Son, that whoever believes in Him shall not perish, but have eternal life. For God did not send the Son into the world to judge the world, but that the world might be saved through Him." (John 3:16-17)
These are Jesus' words to a world of sinners in need of His salvation. The problem unbelievers have is with the words of Jesus, not with Believers who believe and receive the saving life of Jesus (Rom 5:10).