What did Paul mean by saying he could not speak to the Corinthians as unto spiritual?
Paul meant the Corinthian believers had not advanced into the deep things of God. They were still carnal, needing basic, rudimentary instruction rather than spiritual teaching. Unlike the Thessalonians, who quickly grasped the whole gamut from foundational truths to deeper matters, the Corinthians could not yet receive deeper spiritual truth.
Les explains that 1 Corinthians 3 continues the problem he introduced: the Corinthians had more problems than any of the other congregations Paul dealt with. Paul says he could not speak to them as unto spiritual, but as unto carnal. In other words, they had not gone into the deep things. Though they had not had that long an opportunity, Les contrasts the Thessalonians, where Paul was only a very short time, yet could write that they knew these things, because he had covered the whole gamut from the rudimentary to the very deep things. The Corinthians, by contrast, did not.