Matthew McMahon's "Simple Overview of Covenantal Theology" argues that Acts 2:39 ("promise is for you and your children ...") is not meant for Gentiles, but Jews. Peter is showing how the Abrahamic covenant is being fulfilled for Jews and carries on in NT preaching. Ideas of Gentile inclusion into covenant comes later, but this is not an explicit text for it. If true, how can this be used as a proof-text for covenant theology? Doesn't this hurt the RCT argument?
I haven't read CMM's treatment, or else it's been a long time since I did. In any case, the pertinent question would be: to whom does the language, "and to all who are afar off," refer? Interpretively, the obvious first audience is primarily Jewish by birth, however there is a significant contingent of foreign-born coverts to Judaism, plus "adherents," and other alien visitors. Still, if the main target of Peter's sermon are those familiar with the Jewish faith, and especially interested or contracted to the Abrahamic promise, esp. Gen.17:7, which contains the like terms, "I will establish my covenant {the promise}... to be God to you and to your offspring."
The question of "who are all those afar off," is most resonant with the context of all those foreign-born Jews and other parties interested in coming as close as they dared to Jehovah-of-Israel. The promise to Abraham all along has included hope for the nations, "In you all the families of the earth shall be blessed," Gen.12:3. The question has always been, "how" to be IN Abraham, that is, how to benefit from the mediatorship of the divinely instituted covenant relationship. The covenant (in other words) was never solely and only for Abraham's immediate house, however close one wanted to draw the line; but for the whole world. The world was invited to join IN Abraham, then Isaac, then Jacob, then Israel the nation.
CMM could be reflecting on the fact that in the opening bloom of the new-covenant people, it still appears as a "Jewish-movement." The first hearers are primarily Jewish by religion, are familiar with the Abrahamic promise, and so the words of Peter are tailored for a community of listeners who are steeped in Judaism, and who know their Bibles. Peter, unlike Paul later on, goes directly to a fundamental person and expression of the historic faith in order to preach Christ as the fulfillment of the Genesis, the very beginning of Promise.
So, in this Act.2 context, what is more likely: 1) that "all those who are afar off," refers to the strangers to the Promise who were, are, and WILL BE brought near to the Promise-Fulfillment, as those earlier were brought to the Promise-Waiting; or 2) that "to all those who are afar off," refers to various children not present, while "to your children," refers to the children who are present in Jerusalem that day? If the former, it certainly fits into the context of the Abrahamic promise, and resonates with the Jews who are then looking for this very fulfillment to happen someday.
If CMM argues against the idea that this passage is explicit concerning Gentile inclusion (referring only to Jews who will later-on come to faith), I will have to disagree. Because it seems so clear that the setting itself (cf. Act.2:9-11) militates in favor of a direct reference.
Alternatively, CMM might mean only that Peter may still be thinking that the divinely ordered means of inclusion IN the Lord Jesus Christ includes integration into the covenant-life of the national-church. Thus "IN Abraham" by sign might be a rational entailment, and hardly a new-requirement for a predominately Jewish church; as would be legal adherence to Moses, in most if not all particulars. Peter is simply waving people from the Jews and all nations into the church of Christ.
Personally, I wouldn't be so concerned to restrict Peter's terminology to Jews (of all origins and backgrounds), mainly because I don't think that was LUKE'S intent. I think Luke meant his largely Gentile audience to feel the call of God to Christ right from the opening pages of his treatise.
P.S. A postscript on "... as many as the Lord our God shall call."
Sometimes, it has been claimed that a RCT reading of this text ignores this relevant phrase. I can only attribute this reckoning to an understanding of Abraham's covenant as physical, not spiritual, in essence. Because RCT thinks of Abraham's covenant as essentially spiritual--Isaac is in and Ishmael is out, Jacob is in and Esau is out, etc.--we claim that "as many as the Lord our God shall call" is already implicit (if not explicit, prior to the apostasy of either Ishmael or Esau) in the promise to Abraham.
If the only way (ever) to lay hold of the essential thing concerning the Promise has been by faith, because salvation is by grace, then "as many as the Lord our God shall call," has always been the acknowledged stance of the believer's heart. He hoped in God (or acted unbelievingly), respecting the promise, Gen.17:7, as it directly concerned his children. But the believer has always said, "If it is not to be that I should see my children walking in my faith-footsteps, for reasons hidden from me; yet will I hold to my God, and to the unfailing quality of his promises." "Not my will, but Thine be done." "Though he slay me, yet will I trust him."
So "as many as the Lord our God will call," is no ignored aspect of v.39, so far as RCT is concerned. It is as true now, as it has ever been--to Abraham, to Isaac, to David, and so many others. "These all died IN FAITH..." Heb.11:13.