In Matthew, therefore, the Trinitarian G-d was already established?
Yes, certainly. But not just there of course.
Also, it’s important that when people are discussing the Holy Trinity that they do not separate the Holy Spirit. Muslims (and I suppose people of all Religions) are quick to give us a hard time over calling Jesus God, but rarely ever comment on the Holy Spirit; Who is prevalent in the Old Testament.
Anyways, there’s ample reason to understand that Jesus is God if one accepts the words of Christian Scripture as truth; besides the one passage that you asked about.
Psalm 9:7-8 The Lord abides forever, He has established His throne for Judgement, and He will Judge the world in righteousness.
John 5:22 Moreover, the Father judges no one, but has entrusted all judgment to the Son, 23 that all may honor the Son just as they honor the Father. Whoever does not honor the Son does not honor the Father, who sent him.
Matthew 25:31 “When the Son of Man comes in his glory, and all the angels with him, he will sit on his glorious throne. 32 All the nations will be gathered before him, and he will separate the people one from another as a shepherd separates the sheep from the goats. 33 He will put the sheep on his right and the goats on his left.
34 “Then the King will say to those on his right, ‘Come, you who are blessed by my Father; take your inheritance, the kingdom prepared for you since the creation of the world…41 “Then he will say to those on his left, ‘Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels.
I do however believe the Trinity was made known before Jesus’s coming, but not necessarily understood. Here are some interesting passages to ponder:
9 Then the angel of the Lord told her, “Go back to your mistress and submit to her.” 10 The angel added,
“I will increase your descendants so much that they will be too numerous to count.”…13 She gave this name to the
Lord who spoke to her: “
You are the God who sees me,” for she said, “I have now seen[c] the One who sees me.”
Genesis 3:2 There the
angel of the Lord appeared to him in flames of fire from within a bush. Moses saw that though the bush was on fire it did not burn up…16 “Go, assemble the elders of Israel and say to them, ‘The Lord, the God of your fathers—the God of Abraham, Isaac and Jacob—appeared to me and said: I have watched over you and have seen what has been done to you in Egypt.
Genesis 22:10 Then he reached out his hand and took the knife to slay his son. 11 But the **angel of the Lord **called out to him from heaven, “Abraham! Abraham!”
“Here I am,” he replied.
12 “Do not lay a hand on the boy,” he said. “Do not do anything to him. Now I know that you fear
God, because you have not withheld from me your son, your only son.”
However, as a Jewish person this may not at all be convincing for you; so I must ask what you think of the “Spirit of the Lord.” And whether or not said Spirit of God is God?
1 Samuel 10:10 When he and his servant arrived at Gibeah, a procession of prophets met him; the Spirit of God came powerfully upon him, and he joined in their prophesying.
Isaiah 63:11 Then his people recalled** the days of old,
the days of Moses and his people—
where is he who brought them through the sea,
with the shepherd of his flock?
Where is he who set
his** Holy Spirit** among them,
12 who sent his glorious arm of power
to be at Moses’ right hand,
who divided the waters before them,
to gain for himself everlasting renown,
13 who led them through the depths?
Ezekiel 11:5 Then the** Spirit of the Lord** came on me, and he told me to say: “This is what the Lord says: That is what you are saying, you leaders in Israel, but I know what is going through your mind.
2 Samuel 23: 2 “
The Spirit of the Lord spoke through me;
his word was on my tongue.
3
The God of Israel spoke,
the Rock of Israel said to me
Job 33:4The Spirit of God has made me; the breath of the Almighty gives me life.
Notice how these are being used interchangeably?
Isaiah 48:16 “Come near me and listen to this:
“From the first announcement I have not spoken in secret;
at the time it happens, I am there.”
And now the Sovereign Lord has sent me,
endowed with his Spirit.**