1 Corinthians 3:2 (NKJV)
2 I fed you with milk and not with solid food; for until now you were not able to receive it, and even now you are still not able;
Keep in mind the context of Paul’s letter. There is a problem with pride, leading to contentions, in the church.
When Paul first arrived in Corinth, in Acts 18, he spent eighteen months teaching and instructing; giving this baby church milk so they could grow. By now, the church should have been able to chew on solid food, but they were still in the nursing stage of their Christian lives.
Pride stunts our growth. As we humble ourselves, and take our eyes off others and think of the cross of Christ, we can then begin to become more spiritually minded and grow stronger in our Christian walk.