Turn ye even to me with all your heart, and with great fasting, and with weeping, and with mourning. At this period of sore judgment God, by the prophet, calls upon the people to return and repent, to fast and to weep, to grieve inwardly and mourn outwardly for sin.
He also instructs them how to engage in the duty of humiliation aright and acceptably. The humiliation was to be that of the heart—sorrow of heart for the sins by which they had offended God, inward shame on account of those iniquities by which they had wronged their own souls and marred their own best interests.
But while there behoved to be this inward contrition, outward expressions of it were also required. Genuine sorrow and shame for sin were to be accompanied by fasting, tears of penitence, and other indications of mourning.
With all your heart. Kimchi comments thus: "That your repentance be not with a heart and a heart."