The epistles of the New Testament speak of teachings or doctrines that are called mysteries.  These are not mysterious if one understands the use of the Greek word “musterion.” This word translated into English reveals truths pertaining to the Church.  These teachings were not found in the Old Testament Scriptures because they did not pertain to the nation Israel. These newly revealed teachings found primarily in the Pauline epistles are unique to a particular time in history and for a specific group of believers called the Body of Christ, the Church.  “Of this church I was made a minister according to the stewardship from God bestowed on me for your benefit, that I might fully carry out the preaching of the word of God, that is, the mystery which has been hidden from the past ages and generations; but has now been manifested to His saints, to whom God willed to make known what is the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory” (Col 1:25-27). These doctrines all pertain to Christ and are in fulfillment of His prayer in John 17. His prayer to the Father in Gethsemane just prior to His arrest is the basis for a new relationship or position for believers in the Church Age.  Earlier that evening Jesus had told His disciples that He must leave, but another Helper would come who would reveal to them all Truth.  "When the Helper comes, whom I will send to you from the Father, that is the Spirit of truth, who proceeds from the Father, He will bear witness of Me, and you will bear witness also, because you have been with Me from the beginning” (John 15:26-16:1).  "But when He, the Spirit of truth, comes, He will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come” (John 16:13).  Thus the Holy Spirit made His Advent on the day of Pentecost, permanently indwelt every believer and formed the Church.  “Do you not know that you are a temple of God, and that the Spirit of God dwells in you” (1 Cor 3:16)?  Furthermore, every believer in the Church Age would have the indwelling of the Father and the Son … “that they may all be one; even as Thou, Father, art in Me, and I in Thee, that they also may be in Us; that the world may believe that Thou didst send Me. "And the glory which Thou hast given Me I have given to them; that they may be one, just as We are one; I in them, and Thou in Me, that they may be perfected in unity, that the world may know that Thou didst send Me, and didst love them, even as Thou didst love Me” (John 17:21-23).  But there is more, for Christ asked that we be where He is.  "Father, I desire that they also, whom Thou hast given Me, be with Me where I am, in order that they may behold My glory, which Thou hast given Me; for Thou didst love Me before the foundation of the world” (John 17:24). Therefore, the moment that a person places their faith in Christ, believing that He died for their sins, was buried and rose again, God the Father gives eternal life and imputes His righteousness to that individual and God the Holy Spirit places the new believer in union with Christ through the baptism of the Spirit. “For by one Spirit we were all baptized into one body, whether Jews or Greeks, whether slaves or free, and we were all made to drink of one Spirit” (1 Cor 12:13). This union with Christ is eternal, not based on our efforts, but entirely the work of God in grace.  Thus the Apostle Paul will hammer this Truth home with the words “in Christ, in Him,” nearly 200 times in his epistles.  Why is this mystery of a new position in Christ so important?  Because it becomes the basis of our eternal security (Rom 8:38-39); it gives us the privilege to call God our Father; it guarantees us no condemnation (Rom 8:1); we are a new creation in Christ (2 Cor 5:17) and all the provisions given by the Father to sustain the humanity of Christ while on earth now become ours, so that we can be a testimony of Him to His glory (1 Cor 1:30-31).