The recipients of this epistle were not violently rebelling against God, as the context of the book makes clear. Instead, they were drifting away through neglect, neglecting their great salvation. They did not hate God but were simply allowing their Christianity to rust away by not maintaining or growing in it.
Consequently, their lives, consciences, and hearts were gradually hardening. The intensity of their feelings about right and wrong was slipping away, hardened by the deceitfulness of sin. The heart and conscience adjust to such a hardened state that repentance becomes impossible.
A person in this state becomes inured to sin and no longer cares, accustomed to accepting the undesirable. Sin and its effects are undesirable, yet human nature views sin as desirable due to its enmity toward God. Yielding to human nature accustoms the heart and conscience to sin until indifference sets in.
This hardening of conscience echoes Old Testament descriptions of Israel being stiffnecked or having hardness of heart. It expresses the same phenomenon. For those under the New Covenant, however, it is far more dangerous, representing the ultimate departure from God.