Isaac [2617] wrote On the Holy Trinity and a book On the incarnation of the Lord, writing in a very obscure style of argument and involved language, maintaining that three persons exist in one Deity, in such wise that any thing may be peculiar to each which another does not have, that is to say, that the Father has this peculiarity that He, himself without source, is the source of others, that the Son has this peculiarity, that, begotten, He is not posterior to the begetter, that the Holy Spirit has this peculiarity, that He is neither made nor begotten but nevertheless is from another. Of the incarnation of the Lord indeed, he writes that the person of the Son of God is believed to be one, while yet there are two natures existing in him.

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Footnotes:

[2617] Converted Jew, flourished about 385.