In context to verse 9, verse 8 would be the witness given by men with the Holy Ghost of His death at the cross
for why the 3 Witnesses from Heaven as in the witness of God of His resurrection and ascension is greater than the witness of men on earth by the Holy Ghost of His death at the cross.
1 John 5:6 This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth.
7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. 8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
9 If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son.
Jesus said He cannot bear witness of Himself or else His witness is not true; John 5:31. In Matthew 3:15-17, God the Father bear witness of His Son at His water baptism along with the witness of the Holy Spirit's to make the Father's testimony from Heaven true which is required by His words for any testimony to be true; John 8:17. Paul reconfirms this truth for any witness to be established as true in 2 Corinthians 13:1.
So the reason God can establish a word in creation is because there are 2 other Witnesses with the Son's in establishing a word in creation. The request for man to be created in "our image and after our likeness" supports that Three Witnesses within the One God.
Genesis 1:26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth. 27 So God created man in his own image, in the image of God created he him; male and female created he them.
Indeed, when God judged the people at the tower of Babel, the request to go down and scatter them was in the plural sense and yet when "They" did it, it was as the One God.
Genesis 11:5 And the Lord came down to see the city and the tower, which the children of men builded. 6 And the Lord said, Behold, the people is one, and they have all one language; and this they begin to do: and now nothing will be restrained from them, which they have imagined to do. 7 Go to,
let us go down, and there confound their language, that they may not understand one another's speech.
8 So the Lord scattered them abroad from thence upon the face of all the earth: and they left off to build the city.
If God the Father does not judge any one, then the One God can with the Three Persons/Three Witnesses within that One God ( John 5:22-23 )
So God's commandments on men in establishing a testimony or to judge any one ( Deuteronomy 17:6 & Deuteronomy 19:15 ), it cannot be done by One Person, and so as God commands, so as He does by the Three Persons within the One God.
So God cannot be a One Person God, but a Three Persons as within the One God for that One God to judge or establish a true witness from Himself.