The existence of a Divine Trinity, Father, Son and Holy Spirit, is plainly demonstrated by the Word, especially the following passages:

The angel Gabriel said to Mary, The Holy Spirit will come upon you, and the power of the Most High will overshadow you, so that the holy thing that is born of you will be called the Son of God. Luke 1:35.

Here all three are named: the Most High, who is God the Father, the Holy Spirit, and the Son of God.

When Jesus was baptised, behold, the heavens were opened, and John saw the Spirit of God* coming down like a dove, and alighting upon Him; and a voice from heaven saying, This is my beloved Son, in whom I am well pleased. Matt. 3:16, 17; Mark 1:10, 11; John 1:32.

It is even clearer in the words which the Lord used to the disciples:

Go and make all nations disciples, baptising them in the name of the Father, Son and Holy Spirit. Matt. 28: 19.

Moreover in this passage of John:

Three there are that bear witness in heaven, the Father, the Word and the Holy Spirit. 1 John 5:7.

In addition there is the fact that the Lord prayed to His Father, and spoke about Him and with Him, saying that He would send the Holy Spirit, and this too He did. Moreover, the Apostles frequently named in their Epistles the Father, the Son and the Holy Spirit. These passages show plainly that there is a Divine Trinity consisting of Father, Son and Holy Spirit.

* The Latin has 'the Holy Spirit', but this is corrected in the author's copy.