Lord Kṛṣṇa again therefore appeared as Lord Śrī Kṛṣṇa Caitanya Mahāprabhu at the end of the fifteenth century to revive the same Kṛṣṇa consciousness atmosphere in the human society. Lord Caitanya's special gift to the fallen souls of this age of quarrel and disagreement is to induce the people in general—the religionists, the philosophers and everyone—to take to the chanting of the holy name of Kṛṣṇa.
He informed us that the Absolute Supreme Personality of Godhead can descend also in transcendental sound vibration, and thus, when you chant Hare Kṛṣṇa mantra offenselessly, we immediately contact Kṛṣṇa and His internal energy, and thus we become immediately purified from all dirty things in our heart.
Our conditional life of material existence is due to dirty things in our heart. Originally we are all Kṛṣṇa conscious living beings, but due to our long material association in different species of life, varying to 8,400,000's of different forms of life, we are accustomed to transmigrating from one form of body or another.
In such cycle of birth and death, I, you and every one of us, although originally spirit soul and therefore qualitatively one in constitution with the Supreme Lord Kṛṣṇa, we have identified with this material form of life, subjected to various forms of material pangs, specifically in the shape of birth, death, old age and disease.