The "heart" (as it is portrayed) should not be "put on"...it should be "lived".The Bible absolutely supports peaceful coexistence with unbelievers.
"Be wise in the way you act toward outsiders; the most of every opportunity. 6Let your conversation be always full of grace, seasoned with salt, so that you may know how to answer everyone,"
Put on then, as God's chosen ones, holy and beloved, compassionate hearts, kindness, humility, meekness, and patience,
Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you. If someone strikes you on one cheek, turn to him the other also. If someone takes your cloak, do not stop him from taking your tunic. Give to everyone who asks you, and if anyone takes what belongs to you, do not demand it back. Do to others as you would have them do to you.
I Corinthians 13:4-8a, 13 - Love is patient, love is kind, and is not jealous; love does not brag and is not arrogant, does not act unbecomingly; it does not seek its own, is not provoked, does not take into account a wrong suffered, does not rejoice in unrighteousness, but rejoices with the truth. Love bears all things, believes all things, hopes all things, endures all things. Love never fails. Now abide faith, hope and love, these three; but the greatest of these is love.
For where jealousy and selfish ambition exist, there is disorder and every evil thing. But the wisdom from above is first pure, then peaceable, reasonable, full of mercy and good fruits, unwavering, without hypocrisy. And the seed whose fruit is righteousness is sown in peace by those who make peace.
That "heart" is the "core" balance of any and all information one encounters in his/her relationship with their environment. Perceptive thought/speech/action is the response to that interaction...one which should always be tempered with justice/fairness/honest introspection...difficult, but not impossible. That which you encounter in individuals is the varying degree of ability each has to cope with these circumstances confronting them, and the information(or lack of) they have experienced.