This kind of meditation is found in mystic exercises of several religions. In central Europe (where I live) you very often find paintings like the above one (which is from the medieval St. Stephens Cathedral in Vienna). These paintings show Jesus, whose heart is inflamed by love to all humans. Warmth and light radiate out from the holy heart to all suffering humans, spending them love and peace. This love makes no difference between good and evil, but spreads to all beings like the shine of the sun.

As shown in the picture concentrate your awareness at the middle of your breast (not at the physical heart !!!). Breathing in and out imagine as you inflame the fire of your heart, like using bellows (so as the alchemists candled the flame of the athanor = alchemist stove). The fire of your mystic heart is the fire of love.