Father, Son and Holy Spirit
The Trinity is one God in three distinct persons: Father, Son, and Holy Spirit. The persons are not three gods or three temporary roles. The Son is eternally begotten of the Father; the Spirit is eternally God with the Father and Son. Christian prayer and baptism are Trinitarian because salvation draws people into this divine communion.
Follow the sources
An original library introduction, with sources for further study. This is a starting point rather than a complete treatment or a finished formation lesson.