The Trinity is self evident for those who study God's word.
Belief in Jesus is granted by God, as man are utterly depraved and are unable even to believe, much less persevere in Christ.
Those who are regenerated by God, called into Christ, come to know of the trinity by studying the word.
John 6:37, 44: "All those the Father gives me will come to me... No one can come to me unless the Father who sent me draws them, and I will raise them up at the last day."