Theologians today are in agreement that the Hebrew Bible (O.T.) does not contain a doctrine of the Trinity "and that the New Testament also does not contain an explicit doctrine of the Trinity" How do you defend against that?
The Holy Spirit is one with the Father and the Son, When our Lord Jesus says to His disciples, "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit" (Matthew 28:19). He uses the singular form of name "in the name" not "the names". This is different from talking about three distinct people. For example, talking about Abraham, Isaac and Jacob where God calls them "fathers" or "servants" in the plural form (Deuteronomy 6:10; 9:5,27; 29:13 etc.).
Also the verse "For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one" (1 John 5:7) shows clearly that the Three are One. It was never said about Abraham, Isaac and Jacob or about St Peter, St James and St John that they are one.