Verse 8 clearly tells us who didn't that didn't understand. It was the Jewish rulers who crucified Christ. Therefore, the contrast is not between the saved and the unsaved, but the Jewish rulers who couldn't understand the things of the Spirit of God and the apostles to whom the things of the Spirit was revealed.
12 Now we [the apostles] have received, not the spirit of the world, but the Spirit who is from God, that we [the apostles] might know the things that have been freely given to us [the apostles] by God.
13 These things we [the apostles] also speak, not in words which man’s [the Jewish rulers] wisdom teaches but which the Holy Spirit teaches, comparing spiritual things with spiritual.
14 But the natural man [the Jewish ruler] does not receive the things of the Spirit of God, for they are foolishness to him [the Jewish ruler]; nor can he [the Jewish ruler] know them, because they are spiritually discerned.” (1Co 2:12-14 NKJV)
There is no support for the Calvinist inability doctrine in this passage. In fact, Paul said that the veil is lifted from one's eyes "when one turns to the Lord" and not the other way around (2 Corinthians 3:16).