"Why is there a marriage of Christ and His Church?"
To "every thing there is (an appointed) season, and a time to every purpose under the heaven." (Eccl.3:1) There is nothing that God does that does not have a purpose. (Rom.8:28) There is no "idle" (lazy, barren, useless) words (Matt.12:36) in God's purpose. (Eph.3:1-12) The reason (purpose) for a marriage between Christ and His Church is found in the original marriage of the "first Adam" and Eve. (Gen.2:18-25, 1Cor.15:45)

1. The "first Adam" set the pattern
      The apostle Paul says, that he (the first Adam) "was a figure (a model 3) of Him (Jesus) that was to come and
      interprets Jesus to be the "last Adam." (Rom.5:14, 1Cor.15:45 "First the natural, then the spiritual")
      a. The  first Adam was to be married and to be one with Eve
           Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be
           one flesh." (Gen.2:24-25, Eph.5:31-32)
      b. After marriage they were to be fruitful, multiply, and replenish the earth..."
            God "blessed them, and God said unto them, be fruitful, and multiply, and replenish the earth, and subdue it:
           and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth
           upon the earth." (Gen.1:28-29) 
      c. Subdue...have dominion
          After marriage they were to "have dominion over the fish of the sea, and over the fowl of the air, and over every  
          living thing that moveth upon the earth." (Gen.1:28-29)

           They failed, and fell into sin and did not "subdue" or "have dominion!" (Gen.3:1-24, Rom.5:12-21)

2. Jesus...the "last Adam" is fulfilling and will complete this pattern
      Jesus will be married (become "one flesh"- unity / one) with the Church. (Foreshadowed by Eve. Gen.2:18-25,
       Eph.5:21-33, 1Cor.15:45
       a. Today the Church is "espoused"
           This is similar to our modern understanding of being engaged. (2Cor.11:1-3) Since the Cross, (when Jesus
           gave Himself Eph.5:25 for the Church as Adam did for Eve) we have been preparing for the last two days (two
           thousand years 2Peter.3:8-9) for the Wedding.
                     . 
                      We will be "presented" soon  during the Seventh Seal,
                         and appear (in the last day order of events) married and with child (Rev.12:1-17)
                                                     Review: Eschatology Two, Phase.12, Page 1-2

    b. The fruit of this marriage (Not fulfilled yet)
          From this union / unity (John.17:21-24, Eph.4:1-16) of Christ and His Church there will be the man child.
            (Rev.12:1-17) Review: Phase.12
    c. Then (Christ / last Adam and His Church / Eve) will "subdue...(Not fulfilled yet)
          After marriage they will have dominion (Heb.2:8-9) over the fish of the sea, and over the fowl of the air, and
          over every living that moveth upon the earth,"continuing on from where the "first Adam" and Eve fell from
          in the new earth (Rev.22:1) and the new heaven. This is the purpose of the marriage of Christ and His
          Church.

There will be two groups at the time of the marriage
When the marriage takes places there will be two categories in the Church. There will be a group that is "alive and remain" and there will be the "dead in Christ." The "dead in Christ" (every saint, Old and New Testament) cannot
be part of the Marriage because they will not resurrect until Jesus returns. (Review: Phase.10) However God "has prepared for them a city" (Note: Chart below) Out of the "alive and remain" group there will be (a) Those that refuse
to come to the Marriage. (Luke.14:15-24) (b) Those that are not properly prepared and are refused entrance to the Marriage.(Matt.22:11-14; 25:1-13) and (c) Those that fall away (defect, apostatize to the false Bride / marriage because
they don't have a love for the truth - 2Thes.2:10)

1. The "dead in Christ"
      These are the Old and New Testament saints "together" (Eph.4:1-10, 1Thes.4:4:13-18, Heb.11:13-16, Dan.12:1-2)
      that did not live long enough to be "alive and remain" when the Marriage takes place. These"died in faith not having
      received the promises."
      a. Under the Old Covenant (Testament)
           Under the Old Covenant they were to be "gathered unto their people." The first mention is when " Abraham
           gave up the ghost, and died in a good old age, an old man, and full of years; and was gathered to his people."
             (Gen.25:8-9) Others (Gen.25:17; 35:29: 49:29-33; Num.20:24-26; Deut.32:50) When the beggar Lazarus died,
           he was "gathered" to "Abraham's bosom" where Isaac, Jacob, Aaron and the others where kept before the Cross.
           These are the "captivity" that Jesus  lead "captive" when He arose from the grave (Eph.4:7-10) and took to
           paradise with Him. (Luke.23:43) They are now waiting for the "resurrection at the last day." (John.11:24,
             Dan.12:1-2)

                          They will not resurrect until Jesus returns
                                (This prevents them from being in the marriage)
                                                                 (Review: Phase.10)