1. The Trinity is Antichrist because it implicitly denies that Christians are begotten of God
In the doctrine of the Trinity, there is only one begotten child of God, Jesus, and there will never be more than one begotten child of God. He was, and will forever be, God's "only begotten son." In the doctrine of the Trinity, there is no possibility that you or me can really be God's own son or daughter. This is the spirit of the antichrist. All those who are born again from above by the Spirit of God are begotten children of God and God is truly their own Father. Jesus is "the firstborn of many brothers." When Jesus walked in Galilee and when Jesus suffered on the cross, he was indeed God's only begotten son. No one else had ever been begotten of God except Jesus. But that is no longer true. The very point of God sending his son to die on the cross and rise again was not so that he might remain God's only begotten son but so that he might be the firstborn of many brothers, many begotten sons and daughters of God. The doctrine of the Trinity robs this away from the children of God by having them think that only Jesus will ever be God's own true child.
Every true believer who has been born again of the Spirit is a begotten child of God. It is the lie of Trinity doctrine that only Jesus is really God's own begotten child. It is the lie of Trinity doctrine that believers are not God's own begotten children. It is the same spirit of the antichrist that led those who killed Jesus. They could not bear the notion that a human being could possibly be God's own son. And that same spirit of antichrist cannot bear that many human beings have now become begotten children of God.
A difficulty here is that many people may suppose that such language is motivated by an intention to demonize a doctrine simply for the sake of demonizing a doctrine. But if indeed this doctrine is rooted in the spirit of antichrist, we must recognize that reality. The religious leaders killed Jesus because they could not bear the thought that a human being was God's very own son and it was on this basis that they wanted him dead. The same spirit of the antichrist will then likewise reject the notion that any other human, such as you or me, could possibly be God's own begotten child. But the Scriptures plainly teach us that true believers are those who are begotten of God by the Spirit of God. They have died to that life into which they had been born of natural parents and they were begotten again into a new life where their father is God and God is truly their father just as God was truly Jesus' father. As Peter says, they are "partakers of the divine nature." God is spirit, Holy Spirit. And to be a true child of God means we must be, by nature, a spiritual being. Spirit gives birth to spirit. And that is who you are in indeed you have been born again. You are a begotten child of God and He is truly your own father. The spirit of antichrist would have you believe that only Jesus will ever be a true child of God. And that is the devil's lie. All who have been begotten of the Spirit of God are truly His own children just as baby Jesus was His own child since he had been begotten of the Spirit of God. God makes us his sons and daughters not by some vague undefined notion of "adoption." God makes us truly his sons and daughters by begetting us so that we, like Jesus, are truly begotten of God. If we have been born again, God is truly our own Father because he begat us.
2. The Trinity is Antichrist because it necessarily denies that Christ himself is in us.
Christ himself dwells in believers. God the Father dwells in believers. Jesus promised that the Father and the Son would come to make their home in us. This is because believers have the Holy Spirit in them. The Scriptures testify that believers only have ONE Spirit in them, the Holy Spirit of the Living God. In this way the Father and the Son are in them because the Father and the risen Son are that one Spirit. But in the doctrine of the Trinity, the Holy Spirit is most definitely NOT the Son and the Holy Spirit is most definitely NOT the Father. How then could the Father and Son be abiding in anyone who denies that the Father and Son are that one Spirit? But this is the case in the doctrine of the Trinity since the Holy Spirit is NOT the Father and the Holy Spirit is NOT the Son. In the doctrine of the Trinity, the only possible way that the Father and the Son could possibly be in believers is for the Father and the Son to be separate spirits since neither the Father or the Son is the Holy Spirit. Hence, the Trinitarian must either confess he has three Spirits in him, Father, Son, Holy Spirit, or he must confess that only one Spirit is in him who is not the Father and not the Son and thus he must deny that the Father of the Son is in him since that one Spirit is NOT the Father or the Son. But since the Scriptures testify that we only have ONE Spirit in us, the Holy Spirit, the Trinitarian must therefore deny that Christ is in him since the Holy Spirit is not Christ and Christ is not the Holy Spirit in his doctrine. This is the Spirit of the Antichrist which denies the Father and the Son.