I don't really care that some say the concept of the Trinity is pagan in nature. It's a meaningless statement. Other cultures also believe in the Flood. I don't care about that either. Their beliefs are evidence of nothing.
The Bible never uses the word "trinity" but the concept is clearly in scripture. Here are just a few verses that demonstrate it:
(Isaiah 9:6-7 NIV) For to us a child is born, to us a son is given, and the government will be on his shoulders. And he will be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace. {7} Of the increase of his government and peace there will be no end. He will reign on David's throne and over his kingdom, establishing and upholding it with justice and righteousness from that time on and forever. The zeal of the LORD Almighty will accomplish this.
(Zechariah 12:10 NIV) "And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, and they will mourn for him as one mourns for an only child, and grieve bitterly for him as one grieves for a firstborn son.
(John 14:7-11 NIV) If you really knew me, you would know my Father as well. From now on, you do know him and have seen him." {8} Philip said, "Lord, show us the Father and that will be enough for us." {9} Jesus answered: "Don't you know me, Philip, even after I have been among you such a long time? Anyone who has seen me has seen the Father. How can you say, 'Show us the Father'? {10} Don't you believe that I am in the Father, and that the Father is in me? The words I say to you are not just my own. Rather, it is the Father, living in me, who is doing his work. {11} Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the miracles themselves.
(Matthew 28:18-20 NIV) Then Jesus came to them and said, "All authority in heaven and on earth has been given to me. {19} Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, {20} and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age."
(Daniel 7:13-14 NIV) "In my vision at night I looked, and there before me was one like a son of man, coming with the clouds of heaven. He approached the Ancient of Days and was led into his presence. {14} He was given authority, glory and sovereign power; all peoples, nations and men of every language worshiped him. His dominion is an everlasting dominion that will not pass away, and his kingdom is one that will never be destroyed.
(Acts 15:28-29 NIV) It seemed good to the Holy Spirit and to us not to burden you with anything beyond the following requirements: {29} You are to abstain from food sacrificed to idols, from blood, from the meat of strangled animals and from sexual immorality. You will do well to avoid these things. Farewell.
(Acts 21:10-11 NIV) After we had been there a number of days, a prophet named Agabus came down from Judea. {11} Coming over to us, he took Paul's belt, tied his own hands and feet with it and said, "The Holy Spirit says, 'In this way the Jews of Jerusalem will bind the owner of this belt and will hand him over to the Gentiles.'"
(Hebrews 3:7-11 NIV) So, as the Holy Spirit says: "Today, if you hear his voice, {8} do not harden your hearts as you did in the rebellion, during the time of testing in the desert, {9} where your fathers tested and tried me and for forty years saw what I did. {10} That is why I was angry with that generation, and I said, 'Their hearts are always going astray, and they have not known my ways.' {11} So I declared on oath in my anger, 'They shall never enter my rest.'"
(Hebrews 10:15-16 NIV) The Holy Spirit also testifies to us about this. First he says: {16} "This is the covenant I will make with them after that time, says the Lord. I will put my laws in their hearts, and I will write them on their minds."