It is? Galatians 6:16 is a cryptic reference to an "Israel of God." By logical inference from the surrounding context, I completely agree with you (6:15 discusses about how the ritual of circumcision and the state of uncircumcision are irrelevant), but it's overstating the case by a lot to say "Galatians 6:16 [makes it] Biblically-Mandantory that you believe that God's "Israel" in Prophecy = "THE CHRISTIAN CHURCH."
Yes.
Galatians 6:16 is a cryptic reference to an "Israel of God."
It's not bloody "cryptic", Jude.
Paul prefaces his Benediction to "The Israel of God" by declaring that, under the Cross of Christ, the entire world is crucified to Saint Paul, and that Saint Paul is crucified to the entire world; and that neither Circumcision (Jews) nor Uncircumcision (Gentiles) counts for anything, but only the New Creature (i.e., the Regenerate Christian, of whatever ethnicity).
Good grief. About the only way that Saint Paul could make Galatians 6:16 any less "cryptic" would be if he were to climb up on the shoulders of the Elders of the First-Century Church, beat them liberally about the head with his sandals, and scream in their ears: "HELLO! You Christians, you're the Israel of God, get it? Jews, Gentiles, All of you... The Christian Church is God's Israel!! (WAM!) (WAM!) WAM!)... Oops, sorry, I didn't mean to leave bruises -- but are you getting it yet? The Christian Church, All of you, no matter Race or Ethnicity, is GOD'S ISRAEL!!
By logical inference from the surrounding context, I completely agree with you (6:15 discusses about how the ritual of circumcision and the state of uncircumcision are irrelevant),
Of course. It's Logically inferred from the Text itself, the surrounding context, the tone and direction of Paul's Discourse -- the Argument is Logically Correct. There's nothing wrong with supporting a Logically-Correct Argument.
but it's overstating the case by a lot to say "Galatians 6:16 [makes it] Biblically-Mandatory that you believe that God's "Israel" in Prophecy = "THE CHRISTIAN CHURCH."
No, it isn't. "The New Testament is in the Old concealed, the Old Testament is in the New revealed". Surely you have already encountered this Rule of Reformed Interpretation.
If the New Testament establishes the Christian Church as "God's Israel", then it is appropriate to read that understanding back into Old Testament "Last Days" Prophecies concerning "Israel" in order to attain proper understanding.
"The Old Testament is in the New revealed".
The Church is God's Israel.
'Nuff Said.