One of the bigger stumbling points a non-Christian can have with Christian doctrine is the nature of God. In particular, is Jesus God? Islam wants to honor Jesus, but only recognizes him as a prophet. Mormonism and other quasi-Christian groups make Jesus a “god”, which basically means that Jesus is a created being with special powers, but not God himself in any way.
Orthodox Christianity (not to be confused with the denomination) has taught for centuries that God is a complex being who is triune in nature. This means that there are three distinct beings that form one God. This idea is too much for some. They want God to be logical. The Trinity is not logical.
Consider though what logic is. Logic is our experience of how things work in the observable world. Logic does not apply to everything. Quantum physics in not a part of our normal observable experience and is not logical. God is the Creator of all things and thus outside of the universe, our normal observation, the Laws of Physics and consequently outside of logic. It is logical that God is not logical.
The question, therefore, should not be if God can be triune. The question is whether God has revealed His triune nature in Scripture.
The Gospel of John has the most to say about this topic. It starts,
In the beginning was the Word, and the Word was with God, and the Word was God. All things were made through him, and without him was not any thing made that was made.
And the Word became flesh and dwelt among us, and we have seen his glory, glory as the only Son from the Father, full of grace and truth.
Without a doubt, “the Word” refers to Jesus and the passage asserts this illogical truth: the Word was with God (distinct in essence) and the Word was God (united in essence).
Jesus himself said, “The Father and I are one”, and “Do you not believe that I am in the Father and the Father is in me.” Jesus also points to the miracles he did as proof of his identity.
Jesus also frequently refers to himself as “I am”. For instance, “Truly, truly, I say to you, before Abraham was, I am”. This statement evoked the crowd of Jewish leaders to pick up stones to kill Jesus. They weren’t upset about Jesus claiming some ancient existence. The name God gave himself when speaking to Moses through the burning bush was “Yahweh”–essentially “I am”. Jesus was claiming to be Yahweh in the flesh.
There are many, many more statements, names and actions that equate the Father and Son. It is beyond our capacity to fully comprehend how the Father, Son and Spirit are one. Every heretic has tried to do it, however. It is fact to be accepted not fully comprehended.
The step that God has to help you to make is to get over your own need to fully explain or explain away God. Logic won’t get you there, but the persistent Spirit of God can.