Little oops on the quote. Others have already made the biblical references that the Holy Spirit and the Kingdom of God are within us, but this quote doesn’t refer to that. Rather, it is a reference to 2 Corinthians 4.
But actually, recognizing what Paul doesn’t explicitly says brings up an interesting realization in me: Paul is writing how the gospel is a treasure within ourselves. He’s making a comparison to holding the scripture in earthen vessels.
For centuries, this reference must have been missed, but indeed, we do now know that the people of Paul’s time were actually preserving the scriptures within earthen pottery!
Satan blinds those that "believe not"....
11 Corinthians 4:3 But if our gospel be hid, it is hid to them that are lost: In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, Who is the image of God, should shine unto them.
God blinds some so that they "should not understand."
John 12:40 "He hath blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them."
That was interesting. The hard cases, the beasts of the earth, are blinded by Satan. The other souls, believers and righteous people, are blinded from full truth by God for their protection.