The Trinity is one of the most important doctrines in our understanding of God. The purpose of the doctrine is to enable us to better understand the relation between the Father, the Son, and the Holy Spirit. The nature of God revealed to us reminds us that salvation is dependent upon all three persons being truly God, but being only one God. If God is only a singular person, salvation falls apart biblically. Salvation in Jesus Christ reveals to us that God is Father, Son, and Holy Spirit, three persons, but one being.
Image: Eugenio Hansen, OFS, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons. Image cropped to make into a square.