I was reading through Ignatius’ “Letter to the Magnesians” this afternoon, and toward the end of the letter he says this:

“Be subject to the bishop and one another, as Jesus Christ in the flesh was to the Father and as the apostles were to Christ and to the Father, that there may be unity, both physical and spiritual (12.2; emphasis mine).

In Greek it’s, in part, ὑποταγητε…ὡς Ἰησοῦς Χριστὸς τῳ πατρὶ κατὰ σάρκα (emphasis mine).

Here we have what I’d call an early – very early – attestation to the “form of a servant / “form of God” hermeneutic that the later pro-Nicenes would use to refute the Arians, Eunomians, and other anti-Nicenes.