The Mystery of the Holy Trinity

What is the doctrine of the Trinity? The Athanasian Creed, in common use around the sixth century, formulates it this way: "We worship one God in the Trinity and the Trinity in unity, without either confusing the persons or dividing the substance; for the person of the Father is one, the Son's is another, the Holy Spirit's another; but the Godhead of the Father, Son and Holy Spirit is one, their glory equal, their majesty coeternal."