Oneness apologists are constantly developing new arguments against the doctrine of the Trinity, to which Christians should know how to respond (see “Footloose Theology of Roger Perkins” pertaining to the adjective heis and John 10:30.
One such argument states that in the NT, the Greek masculine adjective heis (“one”) always means “one person.” Thus, passages such as Mark 12:29; 1 Tim. 2:5; etc. teach that God is “one person.” However, the Oneness-unitarian so-called semantic rule is clearly refuted by the fact that heis appears in Galatians 3:28 referring to many:
“There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you [humeis, 2nd per. plural] are [este, plural verb] all one [heis] in Christ Jesus.”
Further, this Oneness notion completely backfires at 1 Corinthians 8:6, which contains a double usage of heis. If heis always means “one person,” as Oneness advocates argue, then, the “one [heis] God, the Father” is one person and the “one [heis] Lord, Jesus Christ” is another (distinct) divine person, which is consistent with Trinitarianism, not Oneness:
“yet for us there is but one [heis] God, the Father, from whom are all things and we exist for Him; and one [heis] Lord, Jesus Christ, by whom are all things, and we exist through Him” (cf. Deut. 6:4)