As Christians, we don’t believe in One God (period)…
I think we do, and struggle to understand as we would like to understand.
We believe in One God…(plus)…made up of Three Distinct Persons aka the Holy Trinity.
One Nature/Essence, three divine persons or hypostases. A nature is what one is, while a person is who one is.
We can’t just say ‘One God’ and leave it at that or else other monotheists will think we believe in a single entity as God as they do, but we don’t.
But we do say One God, What they believe I have no idea. If its not what we believe I resort to prayer and attempt conversation where possible.
We always add to ‘One God’ otherwise people will misunderstand us to literally mean One God (period) as the Jews & Muslims believe.
I’ll give you this one, and we can both imagine what it would sound like to somebody who never heard anything such as. This though is an age old question and from the OT in regards to the HS.
Isa 63:11 Then he remembered the days of old, Moses, and his people, saying, Where is he that brought them up out of the sea with the shepherd of his flock? Where is he that put his holy Spirit within him?
Psalm 51:11 Cast me not away from thy presence; and take not thy holy spirit from me.
Baptism-Transfiguration
Matthew 3- And behold a voice from Heaven that said, “This is my Son, The Beloved, in whom I am delighted.
Luke 3 and the Holy Spirit descended on him in bodily form like a dove. And a voice came from heaven: “You are my Son, whom I love; with you I am well pleased.”
Luke 9 A voice came from the cloud, saying, “This is my Son, whom I have chosen; listen to him.”
Mark 9- Then a cloud appeared and covered them, and a voice came from the cloud: “This is my Son, whom I love. Listen to him!”
Psalm 2 I will proclaim the LORD’s decree: He said to me, "You are my son; today I have become your father.
Matthew 12 "Here is my servant whom I have chosen, the one I love, in whom I delight; I will put my Spirit on him, and he will proclaim justice to the nations
John 5 For the Father loves the Son and shows him all he does. Yes, and he will show him even greater works than these, so that you will be amazed.
John 5 And the Father who sent me has himself testified concerning me. You have never heard his voice nor seen his form,
John 12 Father, glorify your name!" Then a voice came from heaven, “I have glorified it, and will glorify it again.”
Mark 1- And a voice came from heaven: “You are my Son, whom I love; with you I am well pleased.”
What we, Christians, believe about God is completely unique and different than what Jews & Muslims believe.
Somewhat, that’s one of those varies with mileage.
Just like there is only one humanity yet made up of billions of unique individual persons; we believe God is One yet made up of 3 Unique Individual Persons.
And I believe its self-evident.

Jesus Christ coming to judge the living and dead. And if you add the presence of the HS through St Mary, it certainly ventures into awe if you hadn’t already reached awe.
Our belief is weird and I don’t understand why the Old Testament did not teach this about God since God is unchanging; however, I do believe it.
I hear you, mystery unlocked through prayer and confirmed. Very Biblical and Apostolic.
Of course, since Trinity was never taught before & would have been condemned (& was by those Faithful to the OT Faith when it was revealed), yet it was true, I do wonder what is currently not being taught yet we’ll find out later is true that we would condemn now?
I wonder if Pilot asked himself that or the Romans for the first 300 years.
Does make for an interesting conversation with something to think about though.