Ephesians 2:5

even when we were dead through our trespasses, made us alive together with Christ (by favor you have been saved)
- Ephesians 2:5

To correct the false teaching that Christ would only have favor on the Gentiles after they died, Paul points out that they were all spiritually dead at one time. This is in contrast to the false teacher's use of the prophecy mentioned in Eph. 5:14

Ephesians 2:1

You were made alive when you were dead in transgressions and sins,
- Ephesians 2:1

To correct the false teaching that Christ is only Lord of those physically dead, Paul points out that they were already spiritually dead.  This is in contrast to the false teacher's use of the prophecy mentioned in Eph. 5:14

Ephesians 2:20

being built on the foundation of the apostles and prophets, Christ Jesus himself being the chief cornerstone;
- Ephesians 2:20

False teachers in Ephesus were saying that the apostles, including Paul, did not teach all there was about Christ and salvation because they had not been given full revelation. The Apostles only laid a “foundation” and the Prophets in the church, as well as the false teachers came in later to build on that foundation. The “foundation” here refers to the teachings of the Apostles and Prophets and not the Apostles and Prophets themselves. The false teachers had been misinterpreting the prophecies of the Prophets to support their false teachings (See Eph. 4:8). Paul responds to the false differences between the Apostles and Prophets by saying they both teach the same thing and have laid the same foundation.

“Cornerstone” is an allusion to Isaiah 28:16:

Therefore thus says the Lord Yahweh, "Behold, I lay in Zion for a foundation a stone, a tried stone, a precious cornerstone of a sure foundation... "

The context there is that God had declared to the leadership in Jerusalem at that time, that they should repent or else God would destroy the city and exile them. The leadership in Jerusalem responded by trying to make alliances to avoid God’s judgment. However, God says that His word, what He has declared, is like a cornerstone and cannot be avoided.

The false teachers had told the Ephesians that there was no salvation for the Gentiles on earth and God had declared it in the Scriptures. The Ephesians responded by saying that perhaps God changed his will and those in Christ would be saved on earth. The false teachers answered by pointing to Isaiah 28:16 as proof that when God declares something, it cannot be avoided. His judgements cannot be changed. Paul responds to this by saying that Christ Jesus is the chief cornerstone, implying that salvation through Christ for everyone (Jews and Gentiles) is God’s ultimate will.