The Gentiles you call "certain Gentiles" were already going to synagogues. It makes no sense that Paul would quote Isaiah 49:6 to preach to Gentiles that had already been preached to by Jews.
The Jews didn't know what Paul knew until Paul revealed it to them, but Paul’s first order of business in the synagogue of the Jews, was to preach the gospel of God (Acts 17:3 KJV) which is concerning the WHO of Jesus Christ and that God raised Him from the dead with power (Romans 1:1-4 KJV). He would then stablish them with the gospel of Christ! And yes, the Gentiles (called Greeks in a KJB) were in the synagogues. They were the “them that fear God” (Acts 13:26 KJV). They were seeking a blessing as per Genesis 12:3 KJV. Little did they know, it would be the forgiveness of sins (Act 13:38-39 KJV). It is the preaching of the cross (1 Corinthians 1:18 KJV, 1 Corinthians 1:21 KJV, 1 Corinthians 15:1-4 KJV)!
Romans 11 has nothing to do with Jews. The words "Jew" and "Jews" are nowhere to be found in Romans 11.
You need to compare spiritual things with spiritual! Romans was written at the time of Acts 20 and Paul is most definitely testifying to them and also to the Greeks (Acts 20:21 KJV).
There was no nation of Israel in the first century.
You’re trippin’ . Israel had fallen, but they were not yet cast away for a reason and for a season (Romans 11:12 KJV) Romans 9:1-5 KJV, Romans 11:1-2 KJV, Acts 28:19-20 KJV.
Israel did not fall in the first century.
Romans 11:11 KJV
The mystery Paul was referring to dealt with the Israelites from the 10 Northern Tribes being scattered amongst the pagan Gentiles. Because the New Covenant was promised to these people, the Gospel of Christ was now being preached amongst the pagan Gentiles.
The NC has nothing to do with the Body of Christ (Hebrews 8:8 KJV) and the gospel of Christ was not preached to alien Gentiles: it was preached to the Jew first and also to the Greek (Romans 1:16 KJV). Get your facts straight, preterist.
This was how "all of Israel" became saved.
It doesn’t say they “became saved”: it says they “shall be”! It is yet future (Romans 11:26-27 KJV)! What’s a preterist to do with a future tense? Change it, that’s what! Caught again!
You don't understand the mystery which was kept secret since the world began.
I most certainly do: it’s the hidden wisdom of God that God ordained before the world unto our glory (1 Corinthians 2:6-8 KJV). It’s Paul’s my gospel (Romans 16:25-26 KJV, 1 Corinthians 15:1-4 KJV)! The gospel you say I wouldn’t know if it bit me on the nose. Paul declared it (1 Corinthians 15:1-4 KJV), I received it, am saved by it, stand by it and preach it; which is more than I can say for you. What has happened to you?
You think Paul was the only person who knew about it.
Read the following:
(Matt 13:35 KJV) That it might be fulfilled which was spoken by the prophet, saying, I will open my mouth in parables; I will utter things which have been kept secret from the foundation of the world.
Paul's gospel was not after man. He neither received it of man, neither was he taught, but by revelation of Jesus Christ (Galatians 1:11-12 KJV)!
The twelve didn't know it until Paul went up by revelation (meaning he was told to go) and communicated it unto them (Galatians 2:1-9 KJV).