In the OT the hardening of the heart is ascribed both to the
individual (Pharaoh in Exod. 8: 15) and also to God's action on
the individual (Exod. 10: 1). Paul takes up the latter in Rom.
9: 18 as part of the sovereignty of God which is not arbitrary
or unjust. In the gospels (e.g. Mark 6: 52) the disciples'
failure to understand the significance of Jesus' miracles is
ascribed to ‘hardness’ or stubbornness.