The first verse of the Holy Bible says, "in the beginning ʼĕlôhîym...". Here we have the Hebrew word "ʼĕlôhîym", which is masculine in gender, and plural in number, GOD. Some have argued that the plural does not refer to "plurality of Persons", but to "majesty", as a king or queen would say, "we". This, however is not the case with "ʼĕlôhîym", when used for the One True God of the Holy Bible. In this same chapter in Genesis, we read, "And ʼĕlôhîym said, Let US make man in our image, after OUR likeness" (verse 26). Again, those opposed to the Holy Trinity, argue that here ʼĕlôhîym is speaking to the angels in heaven, or some other heavenly beings. This is nothing but speculation, because no where in the entire Bible, are we told that human beings are created in the "image" of angels, but we are told, that we are created in the Image of God!
In Ecclesiastes 12:1, we read, "Remember now thy Creator in the days of thy youth". Here the Hebrew word for "Creator", is "bârâʼ", which is plural in number, literally, "Creators"! Likewise, in Job 35:10; and Isaiah 54:5, we have "Your Maker", which again in the Hebrew, "ʻâsâh", is in the plural, "Makers". In Passages like Hebrews 2:10, we read of God the Father as Creator of the universe. Then, in places like Hebrews 1:10:13, we read of God the Father addressing Jesus Christ, and ascribing direct Creation to Him, not as merely the "mediator", as some wrongly assume, but as the actual Creator. In fact, in this passage in Hebrews, the Father is applying Psalm 102:25-27, which is addressed to Yahweh as The Creator, directly to the Lord Jesus Christ, thereby declaring that He is Almighty God. The same Hebrew word, "ʻâsâh", in its singular form, is used for the Holy Spirit in Job 33:4, "The Spirit of God has made [âsâh] me". Here we have Creation ascribed to God the Holy Spirit.
In Isaiah 48:16, we have the clearest reference in the Old Testament, to the Holy Trinity. Here the Speaker is none other than the Lord Jesus Christ, Who Speaks as Yahweh. Here also, those who oppose the Doctrine of the Holy Trinity, have sought to remove the Holy Trinity from this passage, by arguing that the speaker here is the Prophet Isaiah, or someone else. However, this is shown to be quite wrong from the passage itself. In verse 12, the Speaker says, "Hearken unto me, O Jacob and Israel, my called; I am he; I am the first, I also am the last"; and the following verse, "Mine hand also hath laid the foundation of the earth, and my right hand hath spanned the heavens: when I call unto them, they stand up together.". Which is impossible for any human being! So, here in 48:16, we have Yahweh as the Speaker, Jesus Christ, Who says, "and now the Lord (Yahweh) has sent Me and His Spirit". Here both Jesus Christ and the Holy Spirit are the two "objects" in the sentence. Bible versions like the King James, render the Hebrew incorrectly, "and now the Lord GOD, and his Spirit, hath sent me.", making the Lord God and Holy Spirit as two subjects! The order of words in the Hebrew, Greek and Latin, all read: "and now the Lord (Yahweh) has sent Me and His Spirit". As do the following Jewish publications:
“And now the Lord God has sent me, and his Spirit” (The Holy Scriptures According to the Masoretic Text, A New Translation. The Jewish Publication Society of America. 5677-1917)
“And now, the Lord Eternal hath sent me, and his Spirit” (Isaac Leeser, The Twenty-Four Books of the Holy Scriptures: Carefully Translated According to the Massoretic Text)
“ and now, the Lord God has sent me, and His spirit.” (The Complete Jewish Bible, with Rashi Commentary, chabad.org)
“I have not spoken in secret from beginning from the time that it was, there am I : and now the Lord Eternal hath sent me, and his spirit.” (Dr A Benisch; Jewish School and Family Bible, Vol. III)
Also in the Dead Sea Scrolls:
“Draw near to me, and hear this: from the beginning I have not spoken in secret; at the time it came to be, I was there. And now the Lord God has sent me and his spirit.” (Martin Abegg Jr., Peter Flint and Eugene Ulrich. The Dead Sea Scrolls Bible)
In the New Testament we read of the Father sending Jesus Christ; “and the Father Himself, Who has sent Me” (John 5:37, and elsewhere); and the Holy Spirit, "“But the Comforter, the Holy Spirit, Whom the Father will send in My Name” (John 14:26, and elsewhere). However, no where do we read of the Holy Spirit "sending" Jesus Christ!
In Matthew 28:19, we have a very clear reference to the Holy Trinity;
"And Jesus came and spoke to them, saying, “All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptise them into the Name of the Father, and of the Son, and of the Holy Spirit".
The Greek here is of great importance. The sentence, "ὸ ὄνομα τοῦ πατρὸς καὶ τοῦ υἱοῦ καὶ τοῦ ἁγίου πνεύματος" (literally, "The Name of the Father, and the Son, and the Holy Spirit), shows, by the use of the definite artice in the Greek, "τοῦ", which is prefixed to each noun (Father, Son, Spirit), shows that They are "distinct" from each other, though not "separate". Here we have Three "Persons", but One Name, where the Greek "ὸ ὄνομα", in in the singular number. The reference to "ὸ ὄνομα" here is no doubt to that which Yahweh gave Moses at the Burning Bush, "And God said unto Moses, I AM THAT I AM: and he said, Thus shalt thou say unto the children of Israel, I AM hath sent me unto you. And God said moreover unto Moses, Thus shalt thou say unto the children of Israel, The LORD [Yahweh] God of your fathers, the God of Abraham, the God of Isaac, and the God of Jacob, hath sent me unto you: this is My Name for ever, and this is my memorial unto all generations." (Exodus 3:14-15). The Name "Yahweh", is the same root meaning of "I am", meaning "the eternal, self-existing". It is this Name, "Yahweh", which the Father, Son and Holy Spirit have as One.
Let no one suppose, that because here we have the "order" of Persons, as Father, Son and Holy Spirit, that somehow this shows their "importance", or "priority". This is not so. In 2 Corinthians 13:14, the "order" is , "The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen"; and Ephesians 4:4-6, it is, "There is one body, and one Spirit, even as ye are called in one hope of your calling; One Lord, one faith, one baptism, One God and Father of all, who is above all, and through all, and in you all."
Soli Deo Gloria!