Quoting Commentary for I Samuel 12:22

“With its sanctions” [29:11]. With an oath. The Holy One commanded an adjuration and an oath. He will not exchange Israel and they will be His people forever, even when they will be evil. God is their God forever, as the verse says, “seeing that the Lord undertook to make you His people” [I Samuel, 12:22]. This means that the Holy One swore that Israel should be His people. The word “seeing that” means to swear, just as the word alah also means to swear.³Bahya, Deuteronomy, 29:11.

Jerusalem is called an inheritance for bringing its sanctity and establishing it forever. Regarding Jerusalem, the prophet said, "His inheritance shall not be forsaken" (Joel 3:20). At first, it is mentioned that God chose Jerusalem for His divine presence and chose Israel as His treasured nation. Afterwards, it is stated that God will watch over this people whom He has chosen in that place, as it is also said, "For the Lord has chosen Zion" (Psalms 132:13), "For the Lord has chosen Jacob" (Psalms 135:4). "For the Lord will not forsake His people" (1 Samuel 12:22). The greatness of Jerusalem is further explained, and it is said, "This is My resting place forever" (Psalms 132:14). The reference to "within the curtains" and "within the walls of the sanctuary" is explained as I have clarified on numerous occasions.

It is possible to understand the word ובאלתו as referring to G’d, meaning “and by means of His oath which He swore to you.” The whole verse then would mean: “you are standing here today ready to enter the covenant and oath of the Lord by means of which He swore that You will be His people and that He will be your G’d.” The verse following would prove that this is the meaning, seeing the Torah writes there: “as He said to you and swore to your fathers.” Further reinforcement of this interpretation is provided by Samuel I 12,22: “seeing that the Lord had agreed (sworn) to make you His people.” Even though all the commentators there understand the word הואיל to be an expression of G’d’s goodwill, grace, I believe the word הואיל in that verse derives from the word אלה, and means “for G’d swore to make you His people.” This does not preclude that the oath itself was an expression of G’d’s goodwill towards the people so that there is no conflict between the two interpretations. Still further proof of the meaning of אלה in our verse is the comment in Tanchuma Nitzavim 3 on Song of Songs 7,6: מלך אסור ברהטים, usually translated as “a king held captive by tresses” (of hair). According to the Midrash the meaning is “a king who is bound by his oath.” The word אסר meaning oath, occurs in Numbers 30,7. G’d’s oath to the patriarchs that He would not exchange the Jewish people for any other nation is what ties Him to Israel. The word רהטים in the verse from Song of Songs which we quoted is a reference to the patriarchs; this word occurs in the record of their life stories both in Genesis 18,7 where the word הבקר is rendered by Onkelos as רהט; as well as in connection with Yaakov in Genesis 30,38. This is why G’d cannot violate His oath to the patriarchs .
You will find that in Ezekiel 20 when the Jewish people challenged the continued validity of the Torah’s commandments by describing themselves as having been freed (sold) by their master (G’d) since they had become exiled, that the prophet did not accept that argument and vehemently pointed out that the basic relationship between G’d and the Jewish people is an eternal bond, that severing it would mean the total demise of the party who severed it. [I have merely summarized the dialogue in Ezekiel 20. Ed.] Basically, Ezekiel’s argument was that as long as the master had not received monetary compensation from the “buyer” of the slave no transaction had taken place at all; the Jewish people still belong to their erstwhile master, i.e. G’d. This is also reflected in the words of Isaiah 52,3: “you have been sold חנם, without price, therefore you will also be redeemed without monetary compensation having to be paid.”