The purpose of my reply is not to convince you but to point out to others the flaws in your logic.
No, Paul is not saying that because they were faithful that “all those who are faithful in Christ are predestined” as you stated. Ephesians 1 makes no such demand is found anywhere. If you believe it is, then please post it. In fact, Eph 1 mentions nothing of any demand in order to be placed “in Christ” other than of it being by God’s choice alone. That Paul was addressing the faithful does not mean their faithfulness was a prerequisite to be in Christ. That is a non sequitur based upon your assumption of how you would like it to read, but not upon the actual text itself.
Ephesians 1:4 -5 confirms you are wrong – they tell us exactly what the basis of v4 was: solely BY CHRIST Himself.
Ephesians 1:4 tells us they were chosen from before the foundation of the world; it does not say they were placed in Christ from the before foundation of the world, meaning that they would later be placed in Christ. Being chosen and being placed in Him were by God’s two separate actions - nothing of themselves. Verse 1:4 tells us that "he has chosen us"; it doesn't say that "we caused Him to choose us", which, were it by our faithfulness, in order to be correct, it would have to say that but doesn't. Therefore, it is/was all done unilaterally by Christ. You really should read the whole chapter to gain a correct understanding, (not to mention that it is all of one complete thought from start to end) given as it completely contradicts your conclusion.
1 Corinthians 1:30–31 states plainly that it is because of God they were in Christ Jesus. The phrase “but of Him” in that verse, means that not only was their placement in Christ, but also that their being chosen in Christ was too, both are by God, nothing by man - they were two different yet directly associated actions by God. In other words, all was by God: He chose them for and He placed them in, Christ. And that is why, vs 31, can truthfully say "He that glorieth, let him glory in the Lord" and not "glorieth in man".
[
1Co 1:30-31 KJV]
30
But of him are ye in Christ Jesus, who of God is
made unto us wisdom, and righteousness, and sanctification, and redemption:
31 That, according as it is written,
He that glorieth, let him glory in the Lord.
[Eph 1:4-5 KJV]
4 According as
he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
5
Having predestinated us unto the adoption of children
by Jesus Christ to himself, according to the good pleasure of his will,