The problem with the arian heresy is that if Jesus were just a man then what you have as the central mystery of christianity is a human sacrifice. a human sacrifice is no better than a chicken or a pig when it comes to atoning for sins.
The problem with the arian heresy is that the Bible clearly teaches that God is one; that the Father is God; that the Son is God, and that the Holy Spirit is God.
We can deny none of the above.
Christians are therefore trinitarian, and that is why, for one example, Jesus told us to baptize believers in the name of the Father, and of the Son, and of the Holy Spirit.