Cattāri Brahmavihārā

Unfortunately, these qualities of the heart are conditional, for they depend on a tender web of beliefs and feelings — belief in justice and the basic goodness of human nature, feelings of trust and affection. When that web breaks, as it so easily can, the heart can turn vicious. We see this in divorce, broken families, and society at large. When the security of our food source — the basis of our mental and material well-being — gets threatened, the finer qualities of the mind can vanish. People who believe in kindness can suddenly seek revenge. Those who espouse non-violence can suddenly call for war. And those who rule by divisiveness — by making a mockery of compassion, prudence, and our common humanity — find a willing following for their law-of-the-jungle agenda.

This is why compassion based only on belief or feeling is not enough to guarantee our behavior — and why the practice of training the mind to reach an unconditioned happiness is not a selfish thing. If you value compassion and trust, it’s an imperative, for only an unconditioned happiness can guarantee the purity of your behavior. Independent of space and time, it’s beyond alteration. No one can threaten its food source, for it has no need to feed. When you’ve had even just a glimpse of this happiness, your belief in goodness becomes unshakable. That way other people can totally trust you, and you can genuinely trust yourself. You lack for nothing.

Purity of heart is to know this one thing.

https://www.dhammatalks.org/books/PurityOfHeart/Section0004.html