Surah 30. Ar-Rum, Ayah 59

For an explanation of God's "sealing" the hearts of such people, see note [7] on 2:7 .31_1 See Appendix II.

When an attitude of obstinate resistance to Truth is adopted, the natural consequence (by Allah's Law) is that the heart and mind get more and more hardened with every act of deliberate rejection. It becomes more and more impervious to the reception of Truth, just as a sealed envelope is unable to receive any further letter or message after it is sealed. Cf. also ii. 7 and n. 31.