Here are some personal thoughts on the Christian concept of
the Trinity (God existing as the Father, Son, and Holy Spirit,
simultaneously). I've developed these thoughts mainly through
conversations with Muslim friends who believe that the
trinity contradicts the one-ness of God. Indeed, there
are many people who believe in God but view the "mystery" of
the trinity as being too difficult to accept. My view
is that, while the trinity is indeed a mystery, it is not
any more mysterious than the omnipresent nature of God, which
is a part of almost all religions.
If a person prays to God in New Jersey, and another prays at the same time in Australia, and another in Indonesia, we believe that God is simultaneously with all three. His ability to be in New Jersey, Australia, Indonesia at the same time is indeed a mystery, but it does not contradict the "one-ness" of God. The same would hold if we took a submarine to the bottom of the ocean, or a rocket to a remote star in the galaxy: God would still be there. This is his "omnipresent" nature, and in fact implies that God can be in an infinite number of places at the same time. If this is possible, it seems an easier task to be in only _two_ places at the same time: Existing as God the Father in heaven, while simultaneously manifesting part of himself as Jesus on earth, restricting this part to all of the physical constraints of being human. Therefore, using a quantitative measure of mystery, one could say that the the concept of the Trinity has a "mystery value" that is less than or equal to the mystery value of the omni-present nature of God. Therefore, the amount of faith required to believe in God is more than sufficient to also believe in his Trinity existence.
That God is omnipresent is itself very mysterious, but also quite natural for humans to accept and understand. Indeed, when we pray---anywhere in the world---we understand that God is able to hear us. Moreover, according to Christianity, God is not only able to be with us, but cares for us and even desires a personal relationship with each one of us. While this is a mystery, life itself is mysterious, and some mysteries are very good.
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