PROOF OF THE HOLY TRINITY INSIDE THE BIBLE

At the following verses of Genesis when God speaks of himself in plural, and at the hospitality of Abraham when the God appeared in the form of three angels, the Lord reveals that He is the God of the Holy Trinity.     

Thus Isaiah who had better knowledge of the Lord than the other Israelites addressed Him with the word Holy three times.

    Genesis 1:26

⁴ Then God said: "Let us make man in our image, after our likeness. Let them have dominion over the fish of the sea, the birds of the air, and the cattle, and over all the wild animals and all the creatures that crawl on the ground."

Genesis 3:22 Then the LORD God said, "Behold, the man has become like one of Us, knowing good and evil; and now, he might stretch out his hand, and take also from the tree of life, and eat, and live forever "—

Genesis 11:7"Come, let Us go down and there confuse their language, so that they will not understand one another's speech."

Isaiah 6:3And one called out to another and said, "Holy, Holy, Holy, is the LORD of hosts, The whole earth is full of His glory."

Genesis chapter 18 :1-15

The Lord appeared to Abraham near the great trees of Mamre while he was sitting at the entrance to his tent in the heat of the day. 2Abraham looked up and saw three men standing nearby. When he saw them, he hurried from the entrance of his tent to meet them and bowed low to the ground.

3He said, “If I have found favor in your eyes, my lord,^a do not pass your servant by. 4Let a little water be brought, and then you may all wash your feet and rest under this tree. 5Let me get you something to eat, so you can be refreshed and then go on your way—now that you have come to your servant.”

“Very well,” they answered, “do as you say.”

6So Abraham hurried into the tent to Sarah. “Quick,” he said, “get three seahs^b of fine flour and knead it and bake some bread.”

7Then he ran to the herd and selected a choice, tender calf and gave it to a servant, who hurried to prepare it. 8He then brought some curds and milk and the calf that had been prepared, and set these before them. While they ate, he stood near them under a tree.

9“Where is your wife Sarah?” they asked him.

“There, in the tent,” he said.

10Then the Lord^c said, “I will surely return to you about this time next year, and Sarah your wife will have a son.”

Now Sarah was listening at the entrance to the tent, which was behind him. 11Abraham and Sarah were already old and well advanced in years, and Sarah was past the age of childbearing. 12So Sarah laughed to herself as she thought, “After I am worn out and my master^d is old, will I now have this pleasure?”

13Then the Lord said to Abraham, “Why did Sarah laugh and say, ‘Will I really have a child, now that I am old?’ 14Is anything too hard for the Lord? I will return to you at the appointed time next year and Sarah will have a son.”

15Sarah was afraid, so she lied and said, “I did not laugh.”

But he said, “Yes, you did laugh.”

Source New American Standard Bible 1995