It is a serious mistake not to recognize that a word can carry a different meaning in a variety of biblical contexts. The individual situation must always be carefully examined.
For example, the term “kingdom” has different connotations under various circumstances. Jesus spoke of the “kingdom of God” that had been in operation under the Mosaic regime (Matthew 21:43). The word in that context is likely used in the sense of “reign,” i.e., God’s reign among the people of physical Israel was going to be transferred to a “new nation,” spiritual Israel, the church (Galatians 6:16; cf. 1 Peter 2:9).
The term “kingdom” may, of course, be employed as a synonym for the church (Matthew 16:18,19). The saints of the first century church were certainly also citizens of the kingdom of Christ (Colossians 1:13; Revelation 1:6, 9).
In the present context, however, the word “kingdom” is used of the final, heavenly phase of the reign of God. Observe that the passage speaks of an “entrance” into that kingdom that is yet to be enjoyed.
There are several other New Testament references that view the kingdom in a similar fashion. For instance, Paul declared that it will be through many tribulations that we must “enter into the kingdom of God” (Acts 14:22).
Later the apostle wrote: “The Lord will deliver me from every evil work, and will save me unto his heavenly kingdom” (2 Timothy 4:18).
So, underline “eternal kingdom” in 2 Peter 1:11, and in your margin reference: Heavenly phase of kingdom; see 2 Timothy 4:18.