Was Ruth a member of the visible covenant (God's covenant people) when she was married to Naomi's son? Or, if she was not, when (at what point) did she become a member of the visible covenant?
Your question touches on proselytes. The O.T. frequently mentions proselytes, as does the N.T. See for example, the account of Pentecost in Acts 2:
And how is it that we each hear them in our own language to which we were born? Parthians and Medes and Elamites, and residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the districts of Libya around Cyrene, and visitors from Rome, both Jews and proselytes. (Acts 2:8-10, New American Standard Bible)
The NASB footnote on Acts 2:10 defines "proselytes" as "Gentile converts to Judaism ."
At least one of the church's first "deacons" was a proselyte:
Therefore, brothers, pick out from among you seven men of good repute, full of the Spirit and of wisdom, whom we will appoint to this duty. But we will devote ourselves to prayer and to the ministry of the word. And what they said pleased the whole gathering, and they chose Stephen, a man full of faith and of the Holy Spirit, and Philip, and Prochorus, and Nicanor, and Timon, and Parmenas, and Nicolaus, a proselyte of Antioch. (Acts 6:3-5, English Standard Version)
Acts 13:43 refers to "devout converts to Judaism," ESV.
By circumcision a non-Jew was to be treated as a full Israelite (Ex. 12:48f).
And the LORD said to Moses and Aaron, "This is the statute of the Passover: no foreigner shall eat of it, but every slave that is bought for money may eat of it after you have circumcised him. (Ex. 12:43-44, ESV)
Male aliens, though uncircumcised, were to be treated well, even with love:
When an alien lives with you in your land, do not mistreat him. The alien living with you must be treated as one of your native-born. Love him as yourself, for you were aliens in Egypt. I am the LORD your God. (Lev. 19:33-34, ESV)
Ruth became a proselyte, a member of God's covenant people. We are simply not told when—the Book of Ruth is not interested in such a detail. For such questions we are shut up to surmise. To say this is not to yield to complete uncertainty, however.
About ten years after the family left to go to Moab Naomi's two sons died, perhaps close to the same time. Probably early in that ten-year period Ruth married into the family. The perfectly good surmise is that during the time Ruth was married to Naomi's son (a span of years) she heard much about Israel's God from her husband and from Naomi. Then, in God's providence, this came to full conscious faith in Yahweh. Was there at any time in the history of God's people to this day a more deep-seated and moving profession of faith than Ruth's: "your people will be my people and your God my God"?
Later, Boaz could speak to her as one who had come to take refuge under the wings of the God of Israel. Ruth had burned her bridges behind her! All because she had "wholeheartedly accepted the God of Israel and gladly became a member of His chosen people" (from the Korte Verklaring commentary on the Book of Ruth).
Finally, note Ruth's solemn oath:
Where you die I will die, and there I will be buried. May the LORD deal with me, be it ever so severely, if anything but death separates you and me. (Ruth 1:17, ESV.)
This was a self-maledictory oath, of which the O.T. records several (e.g., 1 Sam. 3:17; 25:22; 1 Kings 2:23).
Ruth didn't become a member of God's people via some religious ceremony. We're not told of such, nor should we expect to find such in the book. She certainly did not become a member of God's people by virtue of her marriage! In Scripture marriage is not the gateway into the kingdom! It is enough that Ruth came to acknowledgement of the Lord God of Abraham, Isaac, and Jacob.
Thank you for writing. I hope that my comments are of help.
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