Babies can only drink milk because they have not grown enough to eat solid food. Paul uses this image as a metaphor to mean that the Corinthians can only understand easy truths. They are not mature enough to understand greater truths. (See: [[rc://en/ta/man/translate/figs-metaphor]])
It is implied that they are not ready to understand more difficult teachings. Alternate translation: "you still are not ready to understand the harder teachings about following Christ" (See: [[rc://en/ta/man/translate/figs-explicit]])