The phrase spiritual Israel is used in two primary contexts. The first is as a reference to the entire body of Christian believers, in distinction to the political or racial people of Israel. Spiritual Israel is also sometimes used to suggest concepts related to replacement theology, in which the promises directed toward Israel are now given to the Church, instead.
Galatians 6:16 refers to the “Israel of God.” Given how frequently Paul dismisses ethic or national divisions in this same letter (Galatians 3:26; 4:5–7; 6:15), it is unlikely that he encourages such divisions here. Instead, he refers to the readers as being similar to Isaac: they are the “children of promise” (Galatians 4:28). Paul has a spiritual group in mind in Galatians 6:16, not an ethnic one. This reference to spiritual Israel is clear enough, but not every reference by Paul to Israel is spiritual in nature. Some, such as Romans 9:4, are national and literal. The context is key.
There are other places in the New Testament that suggest a “spiritual Israel” in that they echo terms used in the Old Testament to refer to the Israelites. First Peter 2:9 uses the same terminology as Exodus 19:5–6 in reference to Christians. Galatians 3:29 uses the term heirs, as does Isaiah 65:9. All Christians are “fellow citizens” and members of the house of God, according to Ephesians 2:12–13. Romans 10:12 also says the same—there is no national preference with respect to salvation. Just as we become spiritual “sons of Abraham” by faith (Galatians 3:7), so we can be considered “spiritual Israel” when we receive Christ. In the sense that ethnicity and politics have no relationship to salvation, the term spiritual Israel presents no noteworthy problems.
Replacement theology, on the other hand, uses the concept of a “spiritual Israel” differently. Replacement theology essentially teaches that the church has replaced Israel in God’s plan and that the many promises God made to Israel are fulfilled in the Church instead—Old Testament prophecies are allegorized in order to make them applicable to the church. Replacement theology presents major theological problems, because Scripture says that God has not forgotten or changed His promises to Israel (see Romans 11:1–2, 11, 23, 26, 29). Teaching that promotes a “spiritual Israel,” in the sense that the Church is the focus of God’s prophetic promises for Israel, is not biblically valid.