Are the Corinthians actually inside of Paul's beating heart? Of course not. Paul is saying that the Corinthians were in union within him, that he felt in union with them. Why? Because Paul raised up the church there. Paul knew them. He had preached to them the gospel by which they were converted. He had probably counseled them for baptism, though he baptized only the household of Stephanas and a few others.
He had pastored that congregation from the very beginning and was familiar with the people as personalities. He had eaten with them in their homes. He had likely conducted some wedding ceremonies, maybe even some funerals as well. He had preached countless sermons and Bible studies to those people. He had experienced walking in the marketplace with them. Perhaps he had even experienced some persecution with them. He had sung with them, maybe cried with them. He had laid hands on them when they were sick.
Thus, Paul was in union with these Corinthians because he had experienced life with them. So whether he was in Jerusalem or in Spain—even if he had gone to Britain—wherever Paul went, he carried with him the memories of those associations and experiences. When one of their names came to mind because it was mentioned in a letter that somebody sent to him, he immediately thought of the person, and it was almost as if they were right there with them. It was probably so real that Paul said to himself, "I wish I could reach out and touch them and help them."
Are we beginning to understand how God can be, as it were, everywhere at once? We can grasp it because we carry Him wherever we are. And it does not matter where He is either; He carries us with Him wherever He is. The seed of this remarkable, intimate union has been conceived within us because God initiated it, and we responded to it.
Of course, His powers are much greater than ours, and He can focus those powers on us as an individual personality if He so desires. Even as we can be aware of what is going on inside a small, contained area, the powers of His eyes and ears, of His mind, of His creative force and energy are so great that He is aware of anything at any time regardless of distance.
However, what He is really striving for in our lives is for us to be able to be aware of His presence wherever we go, because that is what is vital to our salvation and to our lives right now and the character that is being built in us.