Part of the process of being quickened to a spiritual life, is the fact that at the new birth, God puts the Holy Spirit within the new born babe in Christ, therefore giving him a spiritual faith, that is a fruit of the Holy Spirit Gal 5:22. When we are newly born, we are babes in Christ.
As we are weaned from the milk of the word, we become "mature" believers that can digest the doctrine of Christ.
Isaiah 28:9-10; Whom shall he teach knowledge? and whom shall he make to understand doctrine? them that are weaned from the milk, and drawn from the breasts. For precept must be upon precept, precept upon precept, line upon line, line upon line, here a little, and there a little.
The natural man in 1 Cor 2:14, cannot discern the things of the Spirit, and thinks them to be foolishness. There is no way that the natural man can be described as "a mature believer, that understands the things of the Spirit"