14 But solid food belongs to those who are of full age, that is, those who by reason of use have their senses exercised to discern both good and evil.
This verse comments on what it means to be spiritually mature.
The word “but” introduces a contrast to the spiritually immature.
solid food belongs to those who are of full age [mature],
Mature Christians listen to, study, absorb, and conform to the Word of God. “Solid food” is for the mature. Mature believers need solid food because milk is not sufficient for their stability.
that is, those who by reason of use [habit]
The idea of these words is to constantly and personally appropriate to oneself what God has to say. A habit is something we continually use or apply to our lives. Maturity comes from steady application of principles to experience. It does not come from a particular experience or spiritually isolated events.
Spasmodic use of Scripture is opposite to regular practice of principles. Consistent practice of the principles of the Bible produces maturity.
Use is a principle of growth.
A faculty that we do not use grows weak. Truth repeatedly appreciated and appropriated becomes stronger. Those who appropriate truth to experience develop a sense of spiritual discernment and maturity.