Here is an excellent article in apologetics for those of us who have relatives or friends who do not believe in the Trinity. The biggest argument tends to be the claim that the concept of the Trinity did not exist before the Ecumenical Council of NIcea in 325AD. This link shows that the idea that Jesus Christ is God was around long before then:
https://www.str.org/w/nine-early-church-fathers-who-taught-jesus-is-god
