In a marriage is God the substance? And what is the husband and wife? Kids if any?
What do you mean by substance?
For the sacrament of marriage, the form of marriage is the solemn exchange of vows by a man and woman given at the wedding ceremony and witnessed by a priest. The matter of the sacrament is the mutual consent and covenant to live together as husband and wife and the consummation of the marriage in a physical act of conjugal relations.
Children are the fruit of marriage, along with the good of the spouses.
The scriptures say the two become one. I am speaking in Aristotlean metaphysical terms. For example, God is one substance, his personage is three. Man is one substance and his personage is one.
Man and wife don’t become one substance in the metaphysical sense. Human beings are one substance expressed in one person. The spouses to not join their (Aristotelian) substances. If that were the case, they would be one human being, not two. That’s not the case.
To quote from the old "exhortation before marriage: “Henceforth you will belong entirely to each other; you will be one in mind, one in heart, and one in affections.” Not one in metaphysical being.
A husband and wife are certainly united in a unique way unlike any other human relationship because they complement each other physically, psychologically and spiritually when they love each other deeply. If they have children - which is the normal outcome of their life together - their unity reflects the creative power of the Blessed Trinity. Any other interpretation of marriage is not Christian.
But can the souls change a little and become more alike, something like blood brothers?
No. The idea of soul mates or blood brothers is more of a romantic / emotional idea rather than one that reflects reality. The two do become one, by uniting themselves in the common good of marriage, putting aside and even sacrificing their individual desires in favor of their life in common and their marital unity. But that does not involve an actual unity of souls.
What about the platonic idea of the world of forms? Can a new form be created that is a united “form”?
You left out the most christian aspect: it’s a sacrament, and it’s not only “we pretend it because we believe it” like some protestants think, only a verbal contract; the graces are real. Like with all sacraments and graces, they do not rely on our belief even our believes are very important.
JimG: No. The idea of soul mates or blood brothers is more of a romantic / emotional idea rather than one that reflects reality. The two do become one, by uniting themselves in the common good of marriage, putting aside and even sacrificing their individual desires in favor of their life in common and their marital unity. But that does not involve an actual unity of souls.
In what way the graces act upon us it is not known; what is sure, like with the others sacraments, they take precedence over our believes: a baptized child is baptized even he doesn’t know it. Sacraments do not interact directly and visibly with our free will, but somehow they are there.