You have asked about Ephesians 4:4-6, with the following comment:
"My father is very firm about not approving of any religious organization that does not clearly believe in the Holy Trinity as defined by traditional Christian churches. He is not open to other paths leading to God. I don't want to debate it, but I would like to understand it better."
Ephesians 4:4-6 reads as follows: "There is one body and one Spirit, just as you were called to the one hope that belongs to your call, one Lord, one faith, one baptism, one God and Father of us all, who is above all and through all and in all."
I've written recently about Unity and the Holy Trinity in another "Interpret This" reply. I assume your father sees the trinity here in the words "Spirit," "Lord" and "God ... Father." I don't really disagree with that understanding. We affirm One Presence and One Power. We would agree that that One Power can be experienced and understood in three different ways—as God the Source, as the Christ Presence of God within us, and as the Holy Spirit of God supporting us on our unfolding spiritual journey.
Traditional Christian belief (since the First Council of Nicea in 325 CE) defines three distinct "personalities" of God, all consisting of one divine substance. We would not be comfortable with that understanding; but the difference, while important, is not severe.
Metaphysically, we understand the Trinity to be Mind (God as Source), Idea (the Christ of God that is our true identity) and Expression (the supportive expressions of God as Holy Spirit). And we find in the passage from Ephesians a ringing affirmation of Oneness—Unity—the Allness of God expressing in every way it can support and serve our own innate Oneness with God as the Christ of our being.
I certainly support your sense that there is no need to debate with your father. I hope this helps you to see that there is not really a great gulf between your understanding and his.