Not quite true. These are assumptions as well and it is great if we look at our underlying assumptions. You and I certainly 'came into existence' but this is not true of the Lord Jesus Christ: He had no human father, for instance. "All bets are off" then, as far as what we'd 'assume' on point. We really need then, pedantic verses that tell us plainly what we can and cannot deduce/understand from His birth. Because of that, I believe most every point necessarily must be founded upon scriptures. I agree, btw, with all scriptures that Unitarians use, just not to the degree where I allow those to erase any other biblical truth given. I cannot, for instance, erase John 1:1 or John 20:18. They are both TOO clear for me to accept what to date, has amounted to hand-waving-less-than-academic explanations. The way John 1:1 is written, in all languages, leaves little doubt "both/and" must be understood. John 1:14,18, contextually makes it abundantly clear (as does all of John's writings). Because of said verses, I'm necessarily "Arian" AND "Trinian" thus Triune (Trinitarian).
I'm not arguing, just laying these scripture topics on the table as necessary for discussion (not even necessarily with me, these are the source materials). -Lon