What is the difference between Judah and Israel?

The Bible documents that a remnant of all the tribes of Israel joined themselves to the tribe of Judah: "...out of all the tribes of Israel such as set their hearts to seek the LORD God of Israel came to Jerusalem, to sacrifice unto the LORD God of their fathers" (2nd Chronicles 11:16). Then, in chapter 15:9, we read, "And he gathered all Judah and Benjamin, and the strangers with them out of Ephraim and Manasseh, and out of Simeon: for they fell to him out of Israel in abundance, when they saw that the LORD his God was with him." Thus, we see that the remnant of the ten tribes was added to the tribe of Judah, and in Judah the remnant retained its identity.

After the ten-tribe Israel was defeated, the kingdom destroyed and the people deported, they ceased to exist as a national identity. This corresponds to the prophecy that Jacob, the first Israelite, made: "The sceptre shall not depart from Judah, nor a lawgiver from between his feet, until Shiloh come; and unto him shall the gathering of the people be" (Genesis 49:10).

Long after the ten-tribe Israel had disappeared, God spoke through Ezekiel addressing "the house of Israel" several times in chapter 3, but this "house of Israel" consisted only of Jews; that means all 12 tribes.

Later, when the Church was founded and the Temple still stood in Jerusalem, the Apostle Peter addressed the Jews in Jerusalem as, "Ye men of Israel" (Acts 2:22). In summary, all Jews are Israelites, but not all Israelites are Jews.