In verses 51 and 52 John explains that Caiaphus was prophesying even though he did not realize it at the time. This is background information. (See: [[rc://en/ta/man/translate/writing-background]])
The word "nation" is a synecdoche and refers to the people of the nation of Israel. (See: [[rc://en/ta/man/translate/figs-synecdoche]])
# would be gathered together into one
This is an ellipsis. The word "people" is implied by the context. AT: "would be gathered into one people" (See: [[rc://en/ta/man/translate/figs-ellipsis]])
# children of God
This refers to people who belong to God through faith in Jesus and are spiritually God's children.