Cyprian (210-258 AD) in his Three Books of Testimonies Against the Jews, in Book Two under item #6, "That Christ is God" references John 20:28 along with many other Scriptures as an accumulation of proofs that Christ is God. This writing predates Nicea by 100 years. The full relevant passage may be read here and a quote from the same is given below:
Also in the Gospel according to John: “In the beginning was the Word, and the Word was with God, and God was the Word. Also in the same: “The Lord said to Thomas, Reach hither thy finger, and behold my hands: and be not faithless, but believing. Thomas answered and said unto Him, My Lord and my God. Jesus saith unto him, Because thou hast seen me, thou hast believed: blessed are they who have not seen, and yet have believed...
Also Novatian (200-258 AD), again at least 100 years prior to Nicea, wrote the following in his Treatise Concerning the Trinity - chapter 13 (full text here) says:
Moreover, this Word “was in the beginning with God, and God was the Word.”5101 Who then can doubt, when in the last clause it is said, “The Word was made flesh, and dwelt among us,” that Christ, whose is the nativity, and because He was made flesh, is man; and because He is the Word of God, who can shrink from declaring without hesitation that He is God, especially when he considers the evangelical Scripture, that it has associated both of these substantial natures into one concord of the nativity of Christ? ... and if, finally, the Apostle Thomas, instructed in all the proofs and conditions of Christ’s divinity, says in reply to Christ, “My Lord and my God;” and if, besides, the Apostle Paul says, “Whose are the fathers, and of whom Christ came according to the flesh, who is over all, God blessed for evermore,” writing in his epistles; and if the same apostle declares that he was ordained “an apostle not by men, nor of man, but by Jesus Christ;” and if the same contends that he learned the Gospel not from men or by man, but received it from Jesus Christ, reasonably Christ is God.
And again, later in the same work, Novation says:
And let us therefore believe this, since it is most faithful that Jesus Christ the Son of God is our Lord and God; because “in the beginning was the Word, and the Word was with God, and God was the Word. The same was in the beginning with God. And, “The Word was made flesh, and dwelt in us. And, “My Lord and my God.. And, “Whose are the fathers, and of whom according to the flesh Christ came, who is over all, God blessed for evermore. What, then, shall we say? Does Scripture set before us two Gods? How, then, does it say that “God is one?” Or is not Christ God also? How, then, is it said to Christ, “My Lord and my God? ... let them understand that, from the fact that God is one, no obstruction arises to the truth that Christ also is declared to be God.”