The devil never converted to Christianity, his mind set is still in the Old Testament.

Firstly, the question is flawed, because believing in the Nicene Creed is not the deciding factor that makes someone a Christian. I am a Christian, and I reject the Nicene Creeds. However, let's just stipulate, purely for the sake of argument, that the Nicene Creed is true, and that acceptance of its truth is what makes someone a Christian. From this viewpoint, there could be no doubt that Satan is also fully aware of the truths it contains, and thus by this narrow definition, Satan would be considered a Christian. (Unless you want to argue that Satan does not believe in Christ's divinity -- would anyone argue that?) Now, for arguments sake, let's instead define "Christian" as "anyone who accepts Jesus Christ as their Lord and Savior." By this definition, Satan is NOT a Christian, because the Atoning Sacrifice of Jesus Christ does not save Satan; He is not Satan's Lord and Savior.

I understand the JWs don't believe in the trinity neither do they believe that he was/is God, yet they are Christians, are they not?

I don't believe that believing in the Trinity or Nicean Creed makes a person a Christian, but that's just me. The Devil is not a Christian by any means at all. If anything, he is completely against Christianity and is God's biggest enemy that we know of.

To know and to believe are two different things. Mere knowledge is not belief.

If you have a bible, read James chapter 2. In this chapter James is referring to faith without works being dead faith. He says this in verse 19: "You believe that God is one. You do well; the demons also believe, and shudder. But are you willing to recognize, you foolish fellow, that faith without works is useless?" (New American Standard version) Jesus said: "Not all who call out Lord, Lord, shall be saved, but only those who do the will of my Father in Heaven." The gift of salvation is free to anybody who accepts it. Accepting that gift and coming into the Kingdom of God makes one a Christian. The Nicean creed has nothing to do with it. The creed was a response to all the different sects and 'isticisms' that were emerging in 'the Way' (what we know now as Christianity). As for the trinity - it's argued back and forth, but at the end of the day I don't think it has absolute bearing on your salvation. As long as you acknowledge Jesus as the Son of God who came in the flesh, died on the cross for your sins and was resurrected for your eternal life - and you choose to accept that and live by that - then I wouldn't be too concerned. Live in simple devotion to Christ and all these things will be revealed to you, either in this life or in eternity.

Believing that God is a "Trinity" does not make you a Christian at all. You become a Christian when you repent (turn away from) your sins, and believe with everything in you that Jesus Christ was God in the flesh and that He died for our sins, rose again, and is now seated in Heaven, and that because of that, the wrath of God is canceled against us. Satan believes in the Trinity, but he sure hasn't turned from his sins at all.

No the devil is not a Christian. You must believe that Jesus is the son of God that died on the cross and rose on the 3rd day and took all your sins upon Himself. AND you must ask Him to come into your heart to be your God so you may WORSHIP Him. The devil believes as he knows it is true. But he does not wish to worship Jesus he seeks to be Jesus and be the one worshipped.