Nontrinitarian Christians do not believe in the trinity, they do not believe that Father, Son and Holy Spirit are all equal as one.
These folks believe that only the Father is without beginning, that the Father is greater than the Son in all things, that the Son had a beginning, and he was brought forth at a certain point as "the firstborn of all creation" and "the only-begotten son." They do recognize Jesus as the Jewish Messiah and Redeemer, and believe that after his ascension to Heaven he resumed his pre-human identity, but exalted to God's right hand until the last days.
They do not believe that the Holy Spirit is an actual person, but rather refers to God's power or character depending on the context. They believe the Holy Spirit is God's "active force" that He uses to accomplish His will. Judaism and Islam also reject the concept of the trinity. How would you describe or defend the trinity to these people, and nontrinitarian Christians in particular?