Basic reason is enough to know that when only one person is spoken of, singular personal pronouns are employed, while when a plural number of persons are in view, this is grammatically reflected by the usage of plural personal pronouns. The inspired scriptures are careful in this respect, since they are in every respect accurate and without error. Every letter of the holy scriptures, the Lord taught, is of value, importance, and significance (Matt. 5:17-18, Matt. 22:32). Thus we can and must learn even from the fine details of scripture, not least of which is the sort of pronouns it uses; ordinarily when scripture uses a singular pronoun, it is intentionally communicating to us that a single person is in view, while it denotes multiple persons by using plural personal pronouns.
“If we receive the witness of men, the witness of God is greater; for this is the witness of God which He has testified of His Son. 10 He who believes in the Son of God has the witness in himself; he who does not believe G
od has made Him a liar, because he has not believed the testimony that God has given of His Son. 11 And this is the testimony: that God has given us eternal life, and this life is in His Son. 12 He who has the Son has life; he who does not have the Son of God does not have life.” (1 John 5:9-12 NKJV)
The truth laid out in these verses is simple- he who has the Son has life; he who does not believe in the Son of God, does not have life. Salvation comes through Jesus Christ, the one Mediator between God and man (1 Tim 2:5), and no one comes to the Father except through Him (John 14:6).
To believe that Jesus Christ is the Son of God, then, is manifestly required for salvation. The confession that Jesus is “the Christ, the Son of the living God” is central to the true Christian faith (Matt 16:16).
Yet tragically, many professing Christians deny the Son of God. They do this by embracing Augustinian trinitarianism.
The personhood of the Holy Spirit is something many Christians assume. Because we are well used to the idea of the Trinity being a group of three persons, many people come to the texts of scripture with an a priori assumption that the Holy Spirit is person, and that wherever the Spirit of God is mentioned, that is understood to refer to a distinct person from the Father and the Son.
That the Father is the one God is important to know, for scripture reveals it; but it is important to know not only that these words are true, but what those words mean according to the scriptures. How, after all, can the Father be the “one God” (1 Cor. 8:6), while the Son is also called God (Jn 1:1)?
To answer this question it is important to understand what the term “God” even means in itself. It is a term used very frequently throughout the scriptures, not only for the Supreme God, the Father, the “Lord God Pantokrator” (Rev 4:8), but also for beings as low as men and angels. As Jesus noted in John 10:35 “He [God] called them gods, to whom the word of God came”, speaking of a psalm where the men of Israel were called “gods”. In Psalm 82:1 “God stands in the congregation of the mighty; He judges among the gods”, calling created angels gods. Paul is well aware of this when he writes “For even if there are many called gods, whether in heaven or on earth (as there are many gods and many lords), 6 yet for us there is one God, the Father, of whom are all things, and we for Him; and one Lord Jesus Christ, through whom are all things, and through whom we live.”