I Corinthians 2:9-12
Even so it has been written: Isaiah 64:4: “Eye has not seen and ear has not heard and it has not entered into the heart of mankind what God has prepared for those who love Him.” However, God has revealed those things to us by His Spirit, for the Spirit searches everything, even the deep things of God. As a matter of fact, who among men knows the things of a person except the spirit of the person that is in him? Even so, no one except the Spirit of God knows the things of God. However, we did not receive the spirit of the world but the Spirit that is from God so that we might know the things that have been freely given to us by God.
Since we have believed on the Lord Jesus Christ and received the spirit of God, we can know the deep things of God. However, the Corinthians weren’t able to handle solid food, only milk, because they weren’t meditating on what they had in Christ and weren’t walking out on it. They were still thinking and living by the sinful flesh nature inherited from Adam.
I Corinthians 3:1-3
I, in turn, was unable to speak to you, brothers, as to spiritual but as to fleshly, as to infants in Christ. I gave you milk to drink, not solid food, for you were not yet able to eat it, but neither are you yet able now. In fact, you are still fleshly, because where there is envy and strife among you, are you not fleshly, and do you not walk according to the manner of mankind?
Hebrews 5:12-14
Even though you ought to be teachers by this time, you have need for someone to teach you again the elementary principles of the first declarations of God, and you have come to need milk rather than solid food. Moreover, everyone who partakes of milk is inexperienced in the Word of justice, for he is an infant. However, solid food is for the mature ones, who because of practice have their faculties trained to discern both good and evil.
“Because of practice”! As we practice thinking and living according to what we have in Christ, we become mature and can handle solid food. Then we can understand the deep wisdom of God.