Please do not assume I am asserting anything here; I am merely probing.
Scripture repeatedly says there is only one God. Jews, Muslims, atheists and others find the "trinity" confusing. Even Christians find it confusing. Maybe it sounds confusing because it is confusing. After all, "God is not the author of confusion."
Jesus says, "I and the Father are one. ...the Father is in me, and I in the Father" (John 10:30-38). Where is the Holy Spirit in that equation? Nothing is ever said about the Holy Spirit being in Them and They in the Holy Spirit. How does a Spirit (John 4:24) have a spirit? Romans 8:9 speaks of "the Spirit of God" and "the Spirit of Christ." Philippians 1:19 speaks of "the Spirit of Christ Jesus." Which is it? The Father's spirit? The Son's spirit? Or a separate entity altogether? Are there 3 Spirits? What about where Scripture refers to the 7 Spirits of God (Is. 11:2; Rev. 1:4-5; 3:1; 4:5; 5:6)? Is God 1-in-10 and 10-in-1?
Just because something is "established," "long-standing," and/or "tradition" does not mean it is correct. Dogma can be wrong. Perhaps the "church" has failed to understand Scripture correctly and has imposed their faulty interpretation and understanding upon the Scriptures (like they often do). Maybe the "trinity" developed because it sounds like there are three people, but maybe there are actually only two: Father and Son. Maybe the Spirit is just some aspect of themselves. After all, how can the Holy Spirit be a third person, and yet be referred to as the Father's (ownership) spirit and/or the Son's (ownership) spirit?
I am not saying this is the case. The Trinity is probably the best explanation that man has come up with to explain Yahweh God, but it may also be wrong. We should be aware of and open to that. Whether someone believes in a dichotomy or a trichotomy, I can accept that. But if they reject Jesus as divine, when Scripture says "the fullness of deity" dwelled in Him (and other such statements), then they are engaging in heresy and I reject it.
Something to think about.
We should always have an open mind, willing to consider possible alternatives, but our minds should never be so open that our brains fall out. We need to rightly discern things. Remember, there are over 40,000 Christian denominations, groups, and sects. They cannot all be right, but they can certainly all be wrong. None of them hold a monopoly on the truth. Scripture should always be our ultimate authority, and any teachings from these 40,000 groups should be weighed against Scripture, discerning the difference between the Old and New Covenants, the Gospel, and grace. Regardless of how they were raised, what they were taught, or what they might presently believe, every professing Christian should be willing to change their beliefs in accordance with Scripture if they are found to be wrong, and to grow in maturity and their understanding. If you refuse to do so, you remain stagnant, ignorant and immature.