I never understood why it was called the Father The Son and the Holy Spirit. I always felt Mary should have been included considering she is supposed to be sinless to Catholics and gave birth to the avatar of God.
I feel like there's no balance when women are taken out of the picture. and monotheistic faiths tend to be heavily male based. I never thought God or the creator was a man or woman but more of an it. But even then, in the trinity, I always thought it would make more sense to call it The Father, The Son and the Mother or The Father, The Mother and the Child. But then I guess they wouldn't be so monotheistic. Even then I thought it'd make more sense that way.
I will give my two cents. The idea of the Trinity is largely a product of its time. We are talking about a largely patriarchal society, and thus it was natural to refer to the Trinity in masculine terms. Its the same reason why historically, many gods, and G-d in general, has been referred to in masculine terms. Patriarchal societies create such an idea, and thus humans write in the manner in which they are taught. That changes from culture to culture, and some do promote more of a feminine quality.
The question then becomes, why hasn't it changed? Well, to a point, it has. We still live in a patriarchal society, but that is changing. Within religious studies, the whole idea of G-d being male has also changed. Back in the 70s actually, there was a movement to coin a term for a gender neutral term that could be applied. It failed, but over the decades, it has become much more popular to refer to G-d in non-gender terms.
The reason for that is because of a different understanding of the Bible. While the Bible often refers to G-d in masculine terms, it is realized that the reason for that is largely because of the manner in which men wrote at that time. At the same time, studies have looked at how G-d is referred to in the Bible, and at times, it is seen that G-d is referred to in feminine terms, and sometimes as a mother. More so, different facets of G-d, such as Wisdom, are generally understood as female, and thus there is an argument that G-d contains both female and male qualities, while transcending both.
The Spirit is one that often is ascribed female qualities as well, so there is an argument for some what of a more balanced view.
As for why Mary is left out, she isn't divine, nor is thought to be divine. Her big contribution really begins and ends with having a child. After that, she largely vanishes.
