It seems it would have been clearly stated in the Bible and in the earliest Christian creeds if the doctrine of the Trinity was genuine and central to Christian belief and especially if belief in it was necessary for salvation as many Trinitarians teach. God gave the Scriptures to the Jewish people, and the Jewish religion and worship that comes from that revelation does not contain any reference to or teachings about a triune God. Surely the Jewish people were qualified to read and understand it, but they never saw the doctrine of the Trinity.
25 At that time Jesus answered and said, "I thank You, Father, Lord of heaven and earth, that
You have hidden these things from the wise and prudent and have revealed them to babes.
26 "Even so, Father, for so it seemed good in Your sight.
27 "All things have been delivered to Me by My Father, and no one knows the Son except the Father.
Nor does anyone know the Father except the Son, and the one to whom the Son wills to reveal Him.
28 "Come to Me, all you who labor and are heavy laden, and I will give you rest.
29 "Take My yoke upon you and learn from Me, for I am gentle and lowly in heart, and you will find rest for your souls.
30 "For My yoke is easy and My burden is light." (Matt. 11:25-30 NKJ)
Holy Scripture is perfectly written, it separates the wheat from the chaff.
There are those who hearts are dull, their ears hard of hearing, they eyes cannot see by their own choice:
For the hearts of this people have grown dull. Their ears are hard of hearing, And their eyes they have closed, Lest they should see with their eyes and hear with their ears,
Lest they should understand with their hearts and turn, So that I should heal them.' (Matt. 13:15 NKJ)
God didn't make it easy lest the wrong people escape revealing their true selves in this life. Allowing a child of the devil "sneak in" to the Kingdom would be a crime against eternity, against all the righteous living there.
Lest they repent, the Scripture requires the Holy Spirit to open the eyes and ears. God doesn't open the ears of the devil's children, so they do not hear.
They could "hear" and "see" the truth if they wanted to, without the Holy Spirit. The Holy Trinity is the only explanation for the relationships of the Father Son and Holy Spirit manifest in Scripture, emmeshed in the very fabric of Scripture. These Three Divine Persons appear and act in Scripture that requires the Holy Trinity be True.
In other words, the effects by the Three Divine Persons in scripture would be contradictory if the Holy Trinity is not The EXPLANATION for how these Three Different Persons act in the name of the One God.
43
"Why do you not understand My speech? Because you are not able to listen to My word.
44 "You are of your father the devil, and the desires of your father you want to do. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he speaks a lie, he speaks from his own resources, for he is a liar and the father of it.
45 "But because I tell the truth, you do not believe Me. (Jn. 8:43-45 NKJ)
Jesus Christ taught the Holy Trinity, repeatedly declaring He was God in the flesh, that YHWH God was His Father in a way only true of Him, the Only Begotten Son of God. The Trinity is so central to the Christian faith, all must be baptized in the One NAME (singular) of God, yet Three Persons are named:
19 "Go therefore and make disciples of all the nations,
baptizing them in the name of the Father and of the Son and of the Holy Spirit,
20 "teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age." Amen. (Matt. 28:19-20 NKJ)
28
And Thomas answered and said to Him, "My Lord and my God!"
29 Jesus said to him, "Thomas, because you have seen Me, you have believed. Blessed are those who have not seen and yet have believed."
30 And truly Jesus did many other signs in the presence of His disciples, which are not written in this book;
31
but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name. (Jn. 20:28-21:1 NKJ)
Yes, belief in the Holy Trinity, in the Name of Jesus---that He is as Thomas confessed BOTH Lord and God, that He is the Christ, the Son of God, is essential for salvation.
PS: Its been agreed by Eastern and Western Christians, the Filioque addition doesn't change the essential agreement all the Church has about the Holy Trinity. Even those who reject the Filioque addition (Eastern Orthodox, me) do confess everything we have in the New Covenant of Christ's Blood, is "from the Son". If it were not for His ransom sacrifice, nothing would proceed from the Father to us except Holy Wrath.
As for "Oneness Pentecostals", it denies the Name of Jesus as the Only Begotten Son of the Father. If you do not believe Jesus is the Son of God, you don't have the Father either:
5 Who is he who overcomes the world, but he who believes that Jesus is the Son of God?
6 This is He who came by water and blood-- Jesus Christ; not only by water, but by water and blood. And it is the Spirit who bears witness, because the Spirit is truth.
7 For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one.
8 And there are three that bear witness on earth: the Spirit, the water, and the blood; and these three agree as one.
9 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 God 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.
13
These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.
(1 Jn. 5:5-13 NKJ)