Possible meanings are 1) this is a metaphor meaning that they have been united to Christ. AT: "have become united with Christ" or "belong to Christ" or 2) this is a metaphor meaning that they have become like Christ. AT: "have become like Christ" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The people to whom God has made promises are spoken of as if they were to inherit property and wealth from a family member. (See: [[rc://en/ta/man/translate/figs-metaphor]])