In order for that to be valid, the Gentile church needs to have never been told to have endurance to the end to be saved. Therefore, your interpretation doesn’t work as far as I can tell. Gentiles are also told to have endurance.
Romans 2
7To those who by persistence in doing good seek glory, honor and immortality, he will give eternal life.
2 Timothy 2
10Therefore I endure everything for the sake of the elect, that they too may obtain the salvation that is in Christ Jesus, with eternal glory.
11Here is a trustworthy saying:
If we died with him,
we will also live with him;
12if we endure,
we will also reign with him.
If we disown him,
he will also disown us;
13if we are faithless,
he remains faithful,
for he cannot disown himself.