Trinity

The Trinity is not just a doctrine of God. Among other things, it is a doctrine of our spiritual life, of our interface with the divine life. Consider, for example, the Form of Life Given to Saint Clare by Francis:

Since by divine inspiration you have made yourselves daughters and handmaids of the Most High King and heavenly Father, and have betrothed yourselves to the Holy Spirit to live according to the perfection of the holy Gospel, I will and promise for me and my brothers to always have for you the same loving care and special solicitude I have for them.

The Holy Spirit inspires us to live according to the Gospel, that is the Word of God, the same Word made flesh in Jesus Christ. By living, through the power of the Spirit, according to the pattern of Jesus Christ, we are made children, servants, and friends of the Most High God is the first Source of all.