And so the extent of Christian kindness is not precise. It may be wider or narrower than we think. This is a call for deep self-examination in the light of Holy Scripture and the deceptiveness of our own heart. The depth of Christian kindness. The point here is that Christian kindness is not merely an external change of manners, it is an internal change of heart.
Christian kindness is tenderhearted.
If the heart is hard on the inside and the manners are meek and polite and helpful on the outside, it is not Christian kindness. When your skin is tender, it doesn't take a very hard touch to make it feel pain. When your heart is tender, it is easily affected. It feels easily and quickly. When you stop and think about it, it is remarkable that this is commanded by the apostle.