Introduction:
The phrase "The LORD their God" is a significant designation found throughout the Bible, emphasizing the covenant relationship between God and His people. This title underscores God's sovereignty, faithfulness, and the unique bond He shares with those He has chosen. It is a recurring theme that highlights God's role as the divine protector, provider, and ruler of His people.

Old Testament Context:
In the Old Testament, "The LORD their God" is frequently used to describe the relationship between Yahweh and the nation of Israel. This relationship is rooted in the covenant established with Abraham and reaffirmed through Moses at Mount Sinai. The LORD declares His identity and His expectations for His people, as seen in passages like Exodus 6:7: "I will take you as My people, and I will be your God. Then you will know that I am the LORD your God, who brought you out from under the yoke of the Egyptians."

Throughout the Pentateuch, the phrase is used to remind Israel of their unique status as God's chosen people and their obligation to obey His commandments. In Deuteronomy 4:20, Moses reminds the Israelites, "But the LORD has taken you and brought you out of the iron furnace, out of Egypt, to be the people of His inheritance, as you are today."

The historical books continue this theme, illustrating how Israel's faithfulness or unfaithfulness to "The LORD their God" directly impacts their national fortunes. In 2 Kings 17:7, the fall of Israel is attributed to their sin against "the LORD their God, who had brought them up out of the land of Egypt."

Prophetic Literature:
The prophets often use the phrase to call Israel back to faithfulness and to warn of the consequences of disobedience. In Jeremiah 7:23, God commands, "Obey My voice, and I will be your God, and you will be My people. Walk in all the ways I command you, that it may go well with you."

The prophets also speak of a future restoration where the relationship between God and His people will be fully realized. Ezekiel 37:27 proclaims, "My dwelling place will be with them; I will be their God, and they will be My people."

New Testament Fulfillment:
In the New Testament, the concept of "The LORD their God" is expanded to include all who believe in Jesus Christ, both Jew and Gentile. This is seen in the fulfillment of the new covenant, as described in Hebrews 8:10: "This is the covenant I will make with the house of Israel after those days, declares the Lord. I will put My laws in their minds and inscribe them on their hearts. And I will be their God, and they will be My people."

The apostle Paul reiterates this inclusive relationship in 2 Corinthians 6:16, where he writes, "For we are the temple of the living God. As God has said: 'I will dwell with them and walk among them, and I will be their God, and they will be My people.'"

Theological Significance:
The phrase "The LORD their God" encapsulates the essence of God's covenantal relationship with His people. It signifies His commitment to be their God, providing guidance, protection, and blessing. In return, His people are called to live in obedience and devotion to Him. This relationship is central to the biblical narrative and is ultimately fulfilled in the person and work of Jesus Christ, who establishes a new covenant through His death and resurrection.

Conclusion:
The phrase "The LORD their God" serves as a powerful reminder of God's enduring faithfulness and the call for His people to remain steadfast in their relationship with Him. It is a testament to the divine promise that transcends time, inviting all who believe to partake in the blessings of being God's chosen people.