"Trinity, in Christian doctrine, the unity of Father, Son, and Holy Spirit as three persons in one Godhead. The doctrine of the Trinity is considered to be one of the central Christian affirmations about God" (Britannica.com).
In two of my recent sermons I have alluded to the doctrine of the Trinity. In John 1:1-4 it clearly teaches that The Word, a title for the one we know as Jesus, was with God and was God before the world was made. To put it another way, the author of the book of John claims that Jesus was himself God and present with God. This a profound, mysterious, confusing, and theologically important declaration. Ultimately it is only a declaration that makes sense when seen through the lens of trinitarian theology. When viewed with this theological paradigm in mind, it becomes one of, if not the most, emphatic declaration of Jesus unity with and distinctness from the other persons in the Godhead.
It isn't the only evidence that God is triune in Johannine literature (the books of the Bible traditionally believed to be written by John the disciple) though - there is a lot more. In fact, I wrote a paper on this very subject in college. Below you will find that paper (professor's notes and all) and I encourage you to read it if you're interested in seeing trinitarian evidence in the Gospel of John.