How far had the Corinthians come out of their pagan background?
The Corinthians had come out of paganism to a degree, as the gospel had taken hold enough to separate them from their pagan background. Yet they remained carnal, babes in Christ, with a long way still to go in spiritual growth.
Les explains that the Corinthian church was at a low spiritual level: they were babes in Christ and carnal, not Paul's pride and joy of spiritual growth like the Ephesians or Philippians. Yet the gospel had taken hold of them enough to bring them out of their paganism, and they had separated themselves from their pagan background to a degree. But they were still carnal and had a long ways to go. Paul's love for them was enlarged despite their failures, and he told them they were not straightened in him but in their own innermost being, or heart.
Ye are not straitened in us, but ye are straitened in your own bowels.