Here "elementary principles" refers to the first things people should be able to understand. Alternate translation: "basic truths of God's messages" or "beginnning lessons of God's word"
Truths about God that are easy to understand are spoken of as if they were milk, the only food that infants can take. Alternate translation: "You have become like babies and can drink only milk" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Truths about God that are difficult to understand are spoken of as if they were solid food, suitable for adults. Alternate translation: "milk instead of solid food that adults can eat" (See: [[rc://en/ta/man/translate/figs-metaphor]])