Why is believing in the Trinity necessary for salvation?
“mysterium Christi explicite credi non potest sine fide Trinitatis, quia in mysterio Christi hoc continetur quod filius Dei carnem assumpserit, quod per gratiam spiritus sancti mundum renovaverit, et iterum quod de spiritu sancto conceptus fuerit” (IIa IIae, 2, 8).
The doctrine of the Trinity is the theological account of God’s life with himself and with his creatures that results from a straightforward exposition of the economy of salvation in Scripture: the Son is sent in the Spirit and by the Father to overcome sin and renew the world; we participate in this victorious renewal as we are joined to the Son by the Spirit, and to the Father by the Son. The Trinity is the theological “grammar” underlying this narrative exposition. To lose one is to lose the other.