to confect the Eucharist?
This is referred to by Paul in 1Cor 11:23:
For I received from the Lord what I also passed on to you: The Lord Jesus, on the night he was betrayed, took bread, and when he had given thanks, he broke it and said, “This is my body, which is for you; do this in remembrance of me.” In the same way, after supper he took the cup, saying, “This cup is the new covenant in my blood; do this, whenever you drink it, in remembrance of me.” For whenever you eat this bread and drink this cup, you proclaim the Lord’s death until he comes.
Note that Paul says he received Eucharist “from the Lord,” but, of course, Paul was not present at the Last Supper. He thus obviously received it from somebody in the early Church, but he equates this with receiving it from the Lord, so whomever gave him Eucharist was acting in persona Christi.
Paul does not say he received Eucharist from the Lord. He says what he received he also passed on to them, and follows with several statements about the Last Supper. Paul makes similar statements in the same letter which raise some doubts about the interpretation of that verse that you gave:
1 Cor. 7:10-11 To the married I give this command (not I, but the Lord): A wife must not separate from her husband. But if she does, she must remain unmarried or else be reconciled to her husband. And a husband must not divorce his wife.
Was Paul acting “in persona Christi” in the above quote, since he identifies what he’s saying as being from the Lord? He’s not doing any sacramental acts that a priest would do when acting in persona Christi. Or is he simply handing on a teaching that originated with Jesus, and that is how his statements are from the Lord?
1 Cor 15:3-8 is another example where Paul hands on what he received, and here again, it is a teaching:
3 For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures, and that he appeared to Cephas, and then to the Twelve. After that, he appeared to more than five hundred of the brothers and sisters at the same time, most of whom are still living, though some have fallen asleep. Then he appeared to James, then to all the apostles, and last of all he appeared to me also, as to one abnormally born.
Given these two examples, what Paul received from the Lord would more likely be the statements which follow (“On the night he was betrayed…” etc.), not the Eucharist. Meaning that what he received were teachings originating with Jesus, and this is not a reference to someone acting “in persona Christi” to confect the Eucharist…
Do you know of a more likely example early enough to be the first recorded instance?
Whether the passage refers to the Eucharist itself or the teaching behind it, it came from the Lord to Paul by way of the Church (as there is no reason to believe Paul ever knew Jesus during his earthly life).
Acting in persona Christi is not limited to consecrating Eucharist - this happens with all Sacraments and anytime the Church teaches in Jesus’ name.
Since laypeople may be valid ministers of Baptism (and matrimony in Western theology), acting in persona Christi is not limited to ordained ministers.
Also, beware of looking for specific verbiage to identify the beginning of a particular belief. The word “Trinity” was not around until 180 AD (but, given your profile says you are Unitarian, that is a wholly different issue). Like many other “Christian words”, it nails down certain ideas that were being debated and honed for the sake of clarity. The beliefs pre-existed the clarification. The clarification existed because people tried to challenge existing beliefs.
I think Paul was the first one to write about it in first Corinthians when he describes that very thing as it was passed down to him.