Radak: They will have one heart to fear Hashem. Their hearts will not turn to abomination, rather, only to Hashem. Yonason translates 'a heart that fears.'

Malbim: Until now, the reasons for sin were the Ru'ach and heart. The Ru'ach puts images from the depth of the Nefesh onto the heart; most of them lean to evil ? "Ki Yetzer Lev ha'Adam Ra mi'Ne'urav" (Bereishis 8:21). They are images of envy, pride, vengeance, and desire. This arouses the heart, the power that rules in the Nefesh, and there is an internal war. The images of Sechel and Binah lean to good. The images of the Yetzer ha'Ra lean to evil. He has two hearts, i.e. rulers and conductors. One entices to evil, and one to good. Then you will have only one heart, which is aroused only to good, like the counsel of Sechel and Binah. The new Ru'ach will put on the heart only images of humility, Kedushah and fear of sin.

Radak: It is hard like a rock ? it does not accept rebukes of Musar.

Radak: The heart of Resha'im, which is hard like a rock.

Malbim: It is hard to engrave and on it. It was not prepared to receive Divine impressions and images of Musar and Chachmah.

Rashi: A soft heart that is easily humbled.

Radak: Flesh is soft. Their heart will be soft to accept rebukes of Musar.

Targum Yonasan: A heart that fears ¹ .

Malbim: It is prepared to receive Divine writing and images of Kedushah and spirituality.