Can you imagine what it would be like to serve a two-faced god? What about a three-faced god? While Modalism, the heresy that denies the Trinity, certainly would not maintain that God is dishonest or two-faced, it does hint at a god that seems to suffer from schizophrenia or at least an identity crisis.
Modalism states that God is a single person who is eternal and yet has chosen to reveal Himself in three modes, or forms, to humanity. In the Old Testament God manifested himself as the Father and in the New Testament through the incarnation Jesus Christ and after His ascension the Holy Spirit. Needless to say this flies in the face of the Trinitarian belief that God is one with three distinct persons.
This view becomes problematic when you consider the baptism of Jesus where Matthew provides an eyewitness account of the simultaneous presence of all three persons of the Trinity (Matt. 3:16-17). The Father speaks from heaven, the Son is baptized and the Spirit descends like a dove. Further, Jesus commands us to baptize, in the Name of the Father, Son and Holy Spirit. In the One Name (i.e. God) who exist in three persons (i.e. Father, son and Holy Spirit).
What are the implications of a shape-shifting god? We immediately run into a problem with God’s immutability which in turn strikes at the security we enjoy from a God who is One and unchanging. If God has not eternally been who He is, what is to say that He will not change again or take on another form?