For he chose us in him before the creation of the world to be holy and blameless in his sight. In love he predestined us for adoption to sonship through Jesus Christ, in accordance with his pleasure and will…
Notice that Paul doesn’t say that God chose us to be in Christ. He rather says God chose us in Christ to be holy and blameless. What God chose from the foundation of the world was that whoever is in Christ will be holy and blameless.
Suppose I conduct a conference at which I show a movie clip from The Princess Bride. You choose at the last moment to attend this conference. At the end of the movie clip you raise your hand and ask, “Mr. Boyd, when did you decide that we’d have to watch that silly movie clip,” to which I might respond, “Well, I decided that six months ago.” You then turn around and say, quite accurately, to the whole conference, “Mr. Boyd predestined us to watch this movie clip six months ago.”
But notice, I didn’t predestine that you individually would watch this movie clip. What I predestined is that whoever shows up at this conference would watch this movie clip. Now that you decided (even at the last minute) to be part of this conference, what was predestined for the whole becomes predestined for you. You are part of the “us” who was predestined to watch the clip.
So too, from the foundation of the world God predestined that whoever is in Christ would become holy and blameless in his sight. But he didn’t predestine certain individuals — as opposed to other unfortunate individuals — to be in Christ. This is left up to our choice. Now that you’ve chosen to be in Christ, what was predestined for the group becomes predestined for you. You, with Paul, can say “In Christ WE (who have chosen to believe) were predestined to be holy and blameless…”
I’m convinced this is what Paul is communicating in this passage.