Honest, prayerful study of the Word brings us to spiritual maturity and understanding. But does it not require superior intellectual powers to understand these “deep things of God”? No indeed. Superior intellects among unsaved men are unable to appreciate even the “simple” truths of the Word, for “they are spiritually discerned” (I Cor. 2:14). And as to the “mystery” made known to Paul by the glorified Lord, the Apostle declares that it is now “revealed unto His holy apostles and prophets by the Spirit” (Eph. 3:5).
The mystery is not merely something more difficult to grasp intellectually, for the Apostle specifically states that it is “not the wisdom of this world” but “the wisdom of God” (I Cor. 2:6,7), and that only by the Spirit of God can it be understood and appreciated. This explains why many of the humblest believers rejoice in the mystery and understand it so clearly, while so many great theologians and religious leaders fail to grasp it and keep confusing it with God’s prophesied program regarding the kingdom of Christ.
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.” This is why the Apostle 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 (See Eph. 1:16-19; Col. 1:9,10). source