The Word of God, the Bible, is God breathed or of "the Spirit of God." Proper study in dependence upon the Spirit therefore brings us to spiritual maturity and understanding  that is of "the Spirit of God." It does not require superior intellectual powers to understand these “deep things of God.”

"But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God." (1 Corinthians 2:10 (KJV)

Even the superior intellects among unsaved unregenerate men are unable to appreciate even the “simple” truths of the Word, because “they are spiritually discerned (understood)” (I Cor. 2:14).

And as to “the mystery,” that is God's secret plan from eternity past, which is made known to Paul by the glorified Lord, the Apostle declares that it is now “revealed... by the Spirit” (Eph. 3:5).

The "mystery" of which Paul wrote 12 times in his 13 epistles from Romans to Philemon is not something difficult to grasp intellectually. No, the Apostle Paul specifically states that the knowledge of "the mystery" is “not the wisdom of this world” but rather “the wisdom of God” (I Cor. 2:6-7). It is wisdom that comes only by "the Spirit of God" who can make it revealed, understood, and appreciated.

This then explains why many of the humblest believers rejoice in the mystery and understand it so clearly, and also while so many great theologians and religious leaders fail to grasp it and why they keep confusing "the mystery" program for the Gentile "body of Christ"  that leads to dwelling "eternal in the heaven" (2Cor 5:1b)... with God’s prophesied program regarding "the kingdom" of Christ that was long-promised to Israel on earth.

The "mystery" is not “hard to be understood” because men are slow of mind to understand, but because they are “slow of heart to believe,” because the devil, who “hath blinded the minds of them that believe not” also seeks to keep God’s people from seeing and rejoicing in the truth of the mystery with its riches of grace, its “one body” and its “one baptism” by "one Spirit" into the one "body of Christ" (1Cor 12:13)

This is why the Apostle Paul prayed so fervently that the believers to whom he ministered might be given “spiritual understanding” to take in the glorious message he was commissioned to proclaim.

16 (I Paul) Cease not to give thanks for you, making mention of you in my prayers; 17 That the God of our Lord Jesus Christ, the Father of glory, may give unto you the spirit of wisdom and revelation in the knowledge of him: 18 The eyes of your understanding being enlightened; that ye may know what is the hope of his calling, and what the riches of the glory of his inheritance in (indwelling) the saints, 19 And what is the exceeding greatness of his power to us-ward who believe, according to the working of his mighty power, (Ephesians 1:16-19 (KJV)

9 For this cause we also, since the day we heard it, do not cease to pray for you, and to desire that ye might be filled with the knowledge of his will in all wisdom and