In regard to the doctrine of the Trinity in general the two great heresies which distracted the Church in S. Cyril’s day were Sabellianism and Arianism, the one “confounding the Persons,” the other “dividing the substance” of the indivisible Unity of the Godhead.  Both these opposite errors Cyril condemns with equal energy:  “Do thou neither separate the Son from the Father, nor by making a confusion believe in a Son-Fatherhood Cat. iv. § 8..”  Again he says:  “Our hope is in Father, and Son, and Holy Ghost.  We preach not three Gods:  let the Marcionites be silenced; but with the Holy Ghost through One Son we preach One God.  The Faith is indivisible; the worship inseparable.  We neither separate the Holy Trinity, like some (that is the Arians); nor do we, as Sabellius, work confusion Cat. xvi. § 4.  See the notes on this and the preceding passage..”  “He says not, I am the Father, but the Father is in Me, and I am in the Father.  And again He said not, I and the Father am one, but, I and the Father are One, that we should neither separate them, nor make a confusion of Son-Father Cat. xi. § 16..”