The Call to the Supremacy of God is one of the three proofs that a divinely appointed vicegerent must come with. This means that he always calls towards the Supremacy of God and does not call to the supremacy of the people. The only person who can rule the people is somebody who is appointed by God and calls the people to God.
Narrated Abu Huraira: Allah's Messenger (PBUH & His Family) said, "Whoever obeys me, obeys Allah, and whoever disobeys me, disobeys Allah, and whoever obeys the ruler I appoint, obeys me, and whoever disobeys him, disobeys me." - Sahih Al-Bukhari, Al-Bukhari, Book of Judgments (Ahkaam), Hadith No. 7137
This narration alone makes it clear that Prophet Mohammed (PBUH & His Family) has the authority to appoint a ruler, not the people. He is the Philosopher King or the Divine Ruler.
The appointment of Adam (PBUH) meant that he was the Ruler, the King, the Prime Minister, and the President. He is the one entrusted by God to execute the will of God on Earth. Hence, the concept of the Supremacy of God means that the person who is holding the affairs of humanity has to be somebody chosen, selected, and appointed by God. He is not elected by people or chosen based on people’s subjective self-interest, the interest of a political party, or the basis of any particular group.
We find the concept of the Supremacy of God evident in the Qur'an:
Say, “O Allāh, Owner of Sovereignty, You give sovereignty to whom You will and You take sovereignty away from whom You will. You honor whom You will and You humble whom You will. In Your hand is [all] good. Indeed, You are over all things competent.” - The Holy Qur'an, (Al-Imran) Verse 26
And when your Lord said to the Angels, "Indeed, I will make upon the Earth a caliph.”. - The Holy Qur'an, Chapter 2 (Al-Baqarah), Verse 30
And in the story of Saul in the Holy Qur'an, we see that the believers asked God to appoint for them a king rather than choosing one for themselves:
Have you not seen the eminences of the Children of Israel after Moses when they said to a prophet of theirs, "Send to us a king, and we will fight in the way of God” - The Holy Qur'an, Chapter 2 (Al-Baqarah), Verse 246
And we find in the Bible:
“When you enter the land the Lord your God is giving you and have taken possession of it and settled in it, and you say, “Let us set a king over us like all the nations around us,” be sure to appoint over you a king the Lord your God chooses. He must be from among your fellow Israelites. Do not place a foreigner over you, one who is not an Israelite”. - The Holy Bible, Book of Deuteronomy, Chapter 17, Verse 14 - 15
For kingship belongs to the Lord, and he rules over the nations. - The Holy Bible, Book of Psalms, Chapter 22, Verse 28
A Psalm of David. The earth is the Lord's and the fullness thereof, the world and those who dwell therein. - The Holy Bible, Book of Psalms, Chapter 24, Verse 1
Then Daniel went to his house and made the matter known to Hananiah, Mishael, and Azariah, his companions, and told them to seek mercy from the God of heaven concerning this mystery, so that Daniel and his companions might not be destroyed with the rest of the wise men of Babylon. Then the mystery was revealed to Daniel in a vision of the night. Then Daniel blessed the God of heaven. Daniel answered and said:
“Blessed be the name of God forever and ever,
to whom belong wisdom and might.
He changes times and seasons;
he removes kings and sets up kings;
he gives wisdom to the wise
and knowledge to those who have understanding;
he reveals deep and hidden things;
he knows what is in the darkness,
and the light dwells with him.
To you, O God of my fathers,
I give thanks and praise,
for you have given me wisdom and might,
and have now made known to me what we asked of you,
for you have made known to us the king's matter.”
- The Holy Bible, Book of Daniel, Chapter 2, Verse 17-23
Jesus (PBUH) said: “I seek not my own will but the will of my Father who sent me.” - The Holy Bible, Book of John, Chapter 5, Verse 30
And Jesus (PBUH) came and said to them, “All authority in heaven and on earth has been given to me.” - The Holy Bible, Book of Matthew, Chapter 28, Verse 18
So now this marker of the call towards the “Supremacy of God” has also been fulfilled, as Imam Ahmed Al-Hassan and Abdullah Hashem Aba Al-Sadiq (From Them is Peace) have used the “Supremacy of God” as proof for their truthfulness and they have also encouraged people to go and research. And indeed we did and we found that all Prophets and Messengers called to the Supremacy of God.
No Prophet or Messenger came and implemented socialism or democracy or came up with any other new governing system that is based on people deciding their own affairs.
Every single Prophet and Messenger proclaimed and taught that submitting to the authority of God is the true religion for the whole of mankind in each and every age.