The cult leader is wrong. Yes, the word trinity doesn't exist in the Bible but the concept is clear. I could show dozens of verses but we'll go with just a few:

(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."

(John 14:8-10 NIV) 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.

(John 17:4-5 NIV) I have brought you glory on earth by completing the work you gave me to do. {5} And now, Father, glorify me in your presence with the glory I had with you before the world began.

(Acts 13:2 NIV) While they were worshiping the Lord and fasting, the Holy Spirit said, "Set apart for me Barnabas and Saul for the work to which I have called them."