St. Isidore on Christology

St. Isidore of Pelusium died ca. 436

You yourself would not deny that the true God over all became a man, not changing what He was, yet taking on what He was not, one existing Son in two natures, immutable and unchangeable, both new and eternal, since you have an exceeding number of proofs concerning these issues from our holy father, the great Athanasius. (Letters I, 323 to Pope Cyril of Alexandria PG lxxviii, 369B)

 Guard your heart with all care, therefore, lest you accept at any time one nature of Christ after the Incarnation. (Letters I, to Timothy the Reader 102 PG lxxviii, 252C)

 Christ become man is not a mere man, but God; one Son exists in both natures. (Letters I, Letter to Theodosius PG lxxviii), 409A)