No need of serving a Vaisnava

One has no need of serving a Vaisnava who may have undergone formal initiation but who has never once chanted the Holy Name offenceless. --- Bhaktivinode Thakur

The ordinary meaning of the word "Krishna" is an entity which is different from Krishna. It is something that is enveloped by the deluding energy of Krishna. It is an object which is comprehensible to the other gross senses besides the ear. It is a product of our sensuous perception. We shall not defile the word "Krishna" by accepting this meaning. --- Bhakti Siddanta Sarasvati

Sri Caitanya Caritamrta

Antya -lila Ch 1-verse 101

One has to learn about the beauty and transcendental position of the holy name of the Lord by hearing the revealed scriptures from the mouths of devotees. Nowhere else can we hear of the sweetness of the Lord’s holy name.

Purport
by
AC Bhaktivedanta Swami Maharaj

It is said in the Padma Purana, atah Sri-krishna-namadi na bhaved grahyam indriyaih. Chanting and hearing of the transcendental holy name of the Lord cannot be performed by the ordinary senses. The transcendental vibration of the Lord’s holy name is completely spiritual. Thus it must be received from spiritual sources and must be chanted after having been heard from a spiritual master. One who hears the chanting of the Hare Krishna mantra must receive it from the spiritual master by aural reception. Srila Sanatana Gosvami has forbidden us to hear the holy name of Krishna chanted by non-Vaisnavas, such as professional actors and singers, for it will have no effect. It is like milk touched by the lips of a serpent, as stated in the Padma Purana:

avaisnava-mukhodgirnam putam hari-kathamrtam
sravanam naiva kartavyam sarpocchistam yatha payah