Trinity Tuesday: Baptism and the Doctrine of the Trinity

St. Ambrose, Bishop of Milan, reminds new converts to the Christian faith:
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So you went down (into the water). Remember what you answered: that you believe in the Father, you believe in the Son, you believe in the Holy Spirit. It is not a case of: I believe in a greater, a lesser, and a least. But by the pledge of your own word you are bound to believe in the Son in the same way as you believe in the Father, and to believe in the Spirit in the same way as you believe in the Son; the only exception is that you profess the necessity of belief in the cross of the Lord Jesus alone--On the Mysteries, 5, 28.