What is the oldest instance of a Christian theologian saying that an issue is OK to disagree about?

There's an infamous story from early church history of the dispute over the correct date for celebration of Easter, which got so heated that people even called their opponents heretics over it and attempted to excommunicate them. (See Eusebius, *Ecclesial History* Book V.23-25). On the other hand, the earliest writing I know of to acknowledge faithfulness of Christians on both sides of a debate is the issue of the future millennial kingdom: Justin Martyr says this in approximately 160 AD in his *Dialogue with Trypho* ch.80, where he says that "many good and pious" Christians think otherwise. Is this the oldest example of a Christian author acknowledging legitimate disagreement about some issue? If not, what is the older "acceptable" dispute? (And let's leave out the issues mentioned by Paul in Romans 14 - I'd like to know about sources outside of the Bible itself.)