This "One Supreme Power" is the Originator of the Divine Law through which this whole Universe works in Divine order. Such Divine order can be seen in our day to day life like rising of Sun, Nourishing of all nature born species etc. We can see all such Divine order, all around us. Human mind let's us understand the outer natural world, but human insight makes it possible to experience the Divine Law in nature. This Divine Law is Supreme. This Earth, Heaven & all the Cosmos were created according to that Divine law. This Supreme Divine law can be understood from below analogy:
Every morning, Sun emerges from the eastern horizon & dispels the darkness of the night. With the help of light of the Sun, whole world gets lit up & all creatures are able to sustain themselves. Sun is the source of energy to this world & it's energy nurtures our earth just like a mother feeds her children. Now just imagine, if Sun stops rising in the morning. In such a case all the creatures including us shall come to perish for the want of sunlight & it's energy.
Who has created this Sun & who in this Universe is ensuring that it rises daily from the east to sustain all creatures? Certainly no human being has done this. Sun was created by the Supreme Power called God & only He is ensuring that it rises daily from the east & that all creation is maintained. It all happens according to that Supreme Divine law which is maintained by that "One Supreme Power" called God.
In general God is the faith of human beings that a Supernatural Power exists in this universe who has created us & all other universal forms. He is having supernatural powers, who is omniscient (knows everything), who is omnipresent (who is present everywhere), omnipotent (who is having all the powers & is all powerful), who is Infinite in size, knowledge & powers, one who is imperishable & indestructible. Faith of humans in such a big & great entity having all the powers is known as God.
In Bhagwat Gita Chapter Ten, Lord Krishna is saying that he is the source of all beings & gods. Not even that as per Lord Krishna, all these universes & creations spring from but a spark of His splendor only.
This whole world has it's existence in that Universal Soul (God) only. God is like the ocean & you are like a wave of that ocean. We all are waves of the same Ocean. There is no higher or lower wave. As the waves and the ocean are one, so are you, part & parcel of God only. One's ego only separates one from the God. Just lose your ego & dive yourself in the ocean of Universal Soul.
When a fire is burning, many sparks come out it. Similarly, all these souls have come out from that Universal Soul. "Mundaka Upanisad" explains that only:
"tat etat satyam yatha su-diptat pavakad
visphulingah sahasrasah prabhavante
sa-rupah tathaksharad vividhah saumya
bhavah prajayante tatra caivapi yanti"
The Supreme God is real and eternal. As many thousands of sparks come from a blazing fire, so many different living entities are manifested from the imperishable Supreme God and then again go & merge into Him.
In Bhagwat Gita also God says that this whole world & it's people are nothing else but part & parcel of Him only. Just like clusters of Yarn-Beads are woven on a thread, so is this whole world threaded in Him. So You are nothing but a "Spark of the Divine".
"mattah parataram nanyat
kincid asti dhananjaya
mayi sarvam idam protam
sutre mani-gana iva" (Bhagwat Gita: Chapter Seven verse 7)
"Sri Krishna said:
There is nothing else besides Me,
Like clusters of Yarn-Beads,
Formed by knots on a thread,
All this world is threaded on Me (universal soul)."
So let's believe in that "One Supreme Power" & worship that Supreme Power who is ruling this Universe, someone who is all powerful. One who is all knowledge, one who is imperishable & indestructible. Such a divine entity having all the powers is called GOD.