We need no divine empowerment to be harsh.
It seems to me that we do not need the power of Christ in our lives to treat others hurtfully, unkindly, or dismissively. Christians seem to possess natural resources sufficient for the task of insensitivity. Of course, even carnal Christians are wise enough to masquerade such strictness as a righteous refusal to compromise, but it is actually a walk in the flesh.
It is the ability to express genuine godly and sacrificial love, longsuffering, graciousness, compassion, and gentle understanding that requires the Holy Spirit. Human righteousness is devoid of biblical love and grace. Biblical grace, righteousness, and strength flow from the Holy Spirit.
“Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality, idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who practice such things will not inherit the kingdom of God. But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law” (Gal 5:19-23).