I would contend that to better understand the meaning of ONE is to better understand the meaning of GOD.
For me, it required a series of deep contemplations that ended up in deeper understanding. A more profound understanding of the meaning of ONE.
ONE means EVERYTHING. Every bit of substance, essence, feeling, time, space, conception. Truth. A fully locked reasoning and guaranteed result. Fully controlled. Fully realized. Perfection. Bliss. Lacking nothing. ALMIGHTY.
It does not matter if ONE is as large a trillion universes, or as small as an atom.
Contemplate this first. ONE is EVERYTHING before Creation. EVERYTHING lacks nothing.
The act of Creation reveals that an ALMIGHTY ONE, in His Perfection, in a Perfect and stunningly counterintuitive Act, made Himself eternally alive. Eternally alive to create children to celebrate and contemplate the Perfect Act for eternity.
An ALMIGHTY ONE made Himself into THREE.
ONE Made an eternal sacrifice. Begetting TWO, equal to Him in His Substance, He sacrificed the assurance of His eternal harmony. TWO - in equal Power and Perfection - turned to ONE and Eternally Sacrificed in return, giving up His Own Divine Will to Do only the Will of ONE. Becoming a Sacrificing Son to an Almighty Father.
Eternal Sacrifice is "Love". Something additional to the substance that transcends EVERYTHING, contained in a Third Person, the HOLY SPIRIT.
The HOLY SPIRIT - EVERYTHING + LOVE - given to man so that we may discern the Will of Our Creator.
ONE becomes THREE.
THREE creates man.
Man separates from THREE.
Separated man creates freely without Eternal Sacrifice.
Man creates evil.
TWO becomes man to remove this evil and forever reconcile His creation - man - back to the Truth of ONE.