Satan is a Oneness Believer

Thou believest that there is one God; thou doest well: the devils also believe, and tremble. 

(James 2:19)

Have you ever wondered why Satan is oneness? The answer is simple. Just like all  the rest of the angelic host, When he came into existence He knew only one God. When he answers to God, he sees only one throne. Satan has been watching history unfold since before the Milky Way came into being. He witnessed God creating the heavens and the earth. He watched as stars suddenly appeared at the command of this one God. He watched God create man in his own image and likeness. He also observed holy men of God as they were moved on by the Holy Ghost to write scripture about this one God.
Here are some Scriptures that he knows and believes:

• Deuteronomy 6:4 Hear, O Israel: The LORD our God is one LORD: 
• Isa 44:6 Thus saith the LORD the King of Israel, and his redeemer the LORD of hosts; I am the first, and I am the last; and beside me there is no God. 
• Isa 44:24 Thus saith the LORD, thy redeemer, and he that formed thee from the womb, I am the LORD that maketh all things; that stretcheth forth the heavens alone; that spreadeth abroad the earth by myself; 
• Zec 14:9 And the LORD shall be king over all the earth: in that day shall there be one LORD, and his name one.
• Malachi 2:10 Have we not all one father? hath not one God created us? 
• Mark 12:32 And the scribe said unto him, Well, Master, thou hast said the truth: for there is one God; and there is none other but he: 
• 1 Corinthians 8:6 But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him. 
• Ephesians 4:6 One God and Father of all, who is above all, and through all, and in you all.

I, too, believe these Scriptures - with all my heart. The Oneness of God is the fundamental doctrine of the Bible. On it, hangs all the remaining doctrines found in the Word of God. I encourage you to study it out for yourself. Search the scriptures and let God reveal this precious truth to you, as it is a revelation - and what a glorious one indeed!