So, best I can see, Paul was not talking about the ones he was writing TO being the ones who were building, but about himself and other teachers who were building on the foundation of Christ Jesus.
I see the same kind of thing as I have continued to examine the letter of 1 Corinthians all the way through. Not only that, but if you put it together with the 2nd Corinthians letter there is a continuation in Paul's thoughts.
"Misery loves company." There is ALWAYS someone sneaking in order to destroy the freedom they sniff out. It's no different than how we acted as children. If a brother or sister was having a good time with friends but I was miserable, I couldn't stand it. If you'll think back I'm sure you can remember situations from both sides (being the fun destroyer AND the one who was having fun till your brother/sister messed it up. Here is the mentality: If I don't have this freedom then you can't have it either.
We have already been brought into freedom through Christ, but that doesn't mean we're ENJOYING that freedom. Here is the strategy of the natural mind: Attack the source of the freedom. Sometimes we do it to ourselves, sometimes other believers do it to us because they are caught up in trying to live by law, and sometimes unbelievers do it to us. The result is the same. That is why Paul was like a bulldog as he stood firmly in the message of freedom through Christ.
The good news message of Christ is the declaration of the source of our freedom: Christ (the resurrected, living Christ) and Him Crucified. If you were to examine the barbs you receive on a daily basis you would realize that they target the SOURCE. But what do we do so often? Get caught up in the illusion that we have to defend ourselves!
As for me, I've determined to be a BULLDOG on the reality of Christ. I find that it allows me to Win Friends and Influence People ... yuck, yuck, hahaha! :) NOT! (For you young kids among us, that's the title of a book by Norman Vincent Peele from years ago that has been used in business seminars all around the world.)
As far as the idea that the foundation Paul laid being a transitional thing, I don't think so. We may not see it as clearly today because of all the Christianity out there, but consider what is going on here among us for this is an excellent example. Here, we are continually being founded upon the gospel that Paul preached, and we have spoken of it over and over as being the message of life.
When I talk to others about Christ I am speaking MY gospel to them. It is the same as Paul's, but it is also mine. The point is that the people in MY world see it as MY message. Most Bible "scholars" will dispute that what I teach agrees with the apostle Paul's message, but it really makes no difference in the long run. Those who are not too familiar with the Bible are usually very surprised that such a message would be found in it based on what they have experienced from religion. All they can really comprehend is that this is Jim's viewpoint (even if I show it to them in the Bible). The fact is that my message of good news really comes from CHRIST, so that when anyone receives it as FROM GOD they are receiving it in truth and NOT because I am right.
The gospel of Christ is the only message that is truly valid in this world because it is not about the one who speaks it, but it is about God Himself. Every other attempt to preach "God's message" to the world that is centered around programs or laws or principles, etc, is not about Christ at all. It is about preaching YOURSELF. Dissect any message in the world and you will also discover that it is centered around the person who speaks it. Try it sometime, it's amazing what you will hear!!! :)
Jesus told the Pharisees:
I have come in My Father's name, and you do not receive me; if another shall come in his own name, you will receive him. John 5:43
Preaching Christ doesn't glorify the speaker in any way. It gives me no personal "kingdom" to establish myself by. I'm always left in the same place: IF IT IS HEARD AT ALL IT IS BECAUSE HIS SPIRIT IS MAKING IT REAL TO THE HEARER. I never really know how it's going to be taken ... and that factor leaves me no place to trust my own abilities. That's why you will always notice the tendency for a speaker to add an element in to validate or cause attention to be put upon himself. The truth of the matter is that the gospel goes out and I can't close the sale (part of my earlier "evangelical" training) - it is totally the work of God.
Consider this: each of you are taking the message of Christ to your individual "worlds" ... and TO THEM it is YOUR gospel. Remember who it is that you are telling them about. It is a strange message, and unlike any they have heard. Because it's not about YOU but about HIM.