Lord Jesus Christ reveals his mysteries to his followers only but not to others. "Matt 13:11 He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given. Col 1:26 Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:"  Even though the kingdom principles are kept secret in Old Testament times, God and his eternal justice never changes and they are always the same.  "Eph 3:4 --- the mystery of Christ 5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;  Heb 13:8 Jesus Christ the same yesterday, and to day, and for ever."  Even though the mysteries are revealed in New Testament times, since it is revealed to only a few, they are still mysteries to most Christians.

God, three in one, is still a mystery to most Christians.  The Bible describes God as three Persons knitted in one Spirit as one Person.  Among the three persons the role of God the Word is very special for us.  He was God, who created all things including spiritual powers.  Then he was made (became) flesh (a mere man).  "John 1:14 And the Word was made flesh, and dwelt among us, ---"  As a man, just with the provisions that was available for man, he lived without committing sin.  "1 John 3:5 And ye know that he was manifested to take away our sins; and in him is no sin."  Jesus of Nazareth lived a sanctified life so that his disciples may also live a sanctified life and we all have a calling to follow the steps of his sinless life.  "John 17:19 And for their sakes I sanctify myself, that they also might be sanctified through the truth.  1 Peter 2:21 For even hereunto were ye called: because Christ also suffered for us, leaving us an example, that ye should follow his steps: 22 Who did no sin, neither was guile found in his mouth:" 

Jesus Christ will appear to receive those who will keep themselves pure as he is pure.  When the Lord meets them their appearance will also be changed as Jesus Christ.  "John 3:2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. 3 And every man that hath this hope in him purifieth himself, even as he is pure."

These pure people not only be changed in the appearance, but also they become heirs with the Lord Jesus Christ.  They will sit in his heavenly throne and rule with the Lord.  "Romans 8:17 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together. 18 For I reckon that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us.  Rev 3:21 To him that overcometh will I grant to sit with me in my throne, even as I also overcame, and am set down with my Father in his throne."  The great mystery is that God became a man so that a man may sit with God in his throne.  "2 Cor 8:9 For ye know the grace of our Lord Jesus Christ, that, though he was rich, yet for your sakes he became poor, that ye through his poverty might be rich." 

God Bless You