Eph 1:4

Eph 1:4 says 

just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love, 

The idea that the Ephesians were in Christ before the foundation of the world, and that this proves that all who were and are in Christ were in Christ before the foundation of the world, contradicts Romans 16:7, which says that people were in Christ before Paul. This means that Paul was in Christ after them. This seems impossible if Paul was in Christ before the foundation of the world.

Eph 2:12-13 says the Ephesians were without Christ in the past, but are in Christ now:

that at that time you were without Christ, being aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who once were far off have been brought near by the blood of Christ.

If people were in Christ before the foundation of the world, why would they even need to get regenerated? 

Calvinism might say, the meaning is that people were chosen before the foundation of the world, to be (future tense) in Christ - later, when they get regenerated. However, this seems to be adding to the scripture, to force scripture to agree with ideology, or eisegesis, instead of exegesis, instead of sola scriptura. Also, in this case, people would be chosen when they were not in Christ (to become in Christ in the future). This means they were chosen for salvation outside of Christ, then their salvation was determined before they were in Christ. So the determining factor of their salvation is election, not being in Christ (since it happened when they were not in Christ). Therefore, whether Christ died is irrelevant for their salvation. (They were chosen for salvation before the death, burial and resurrection of Christ, and without Christ; the election is unconditional.) Therefore Christ is irrelevant, and this is not the gospel. Therefore, the idea of people being chosen to be in Christ as the meaning of this scripture, is incorrect.

On the other hand, some might say, we were in Christ before the foundation of the world. In Christ then becomes the condition, violating unconditional election. Also, the question becomes, How did they get out of Christ? For Paul people get into Christ on being born again, or regeneration. To put it another way, how did they - who were in Christ before the foundation of the world - get to the condition of Eph 2:12: "without Christ, being aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world" ? Jesus came to seek those who were lost - how did those already in Him, become lost?

The Calvinistic interpretation as seen fails.

It seems the best interpretation, to agree with scripture, is that God, before the foundation of the world, decided that He those who get into Christ "should be holy and without blame before Him in love." This indicates they were chosen because of a condition, being in Christ, which is contrary to unconditional election.