The paradigm of covenant is union of two which produces a third. It is a triad of communion in love.
Mankind was created in the image of God. That is why God instituted the covenant of the marital union. It is a picture and image of the love union between the members of the Blessed Trinity.
When two are joined, they become one...one flesh. They are no longer two separate beings, but are instead a unit. This is why marriage is supposed to be (in the ideal world before the Fall and sin) indisoluable. And through their union, a third party comes -- the children, who are part of the both. So in the triad which is the family, there is fleshly union between all three members of the covenant. Husband and wife become "one flesh" (according to St. Paul) and then there come others who are indeed made up of both flesh.
And they live in and commune in love as a family.
A picture of the Blessed Trinity.
The Church is just another family. It follows the covenantal norm because God ordained it as a covenantal unit. The OT is filled with references to the covenant. For the kingdom to be covenantal, it must follow the covenantal pattern laid out in scripture. The kingdom of God in Heaven is simply the rulership of the Blessed Trinity over time and space. Therefore, the kingdom of God on earth must follow that same pattern -- three in one, a covenantal union.
Just remember, however, even with this analogy, we are describing the One Who is indescribable. The analogy is given to us, but it is only good to a certain point. The reality of God's love in Blessed Trinity is so far above the analogy that it would blow our feeble minds if we were to see it in reality.
