As stated in Bhagavad-gita (4.8):

Lord Krsna appears in every millennium for two purposes, namely to deliver the devotees and kill the nondevotees. His devotees also have two similar purposes-to preach the bhakti cult of Krsna consciousness and defeat all kinds of agnostics and atheistic demons. Nityananda Prabhu carried out the order of Lord Sri Caitanya Mahaprabhu in this way, and those who strictly follow Nityananda Prabhu perform the same activities. There are two classes of devotees. One is called gosthyanandi, and the other is called bhajananandi. A devotee who does not preach but always engages in devotional activities is called a bhajananandi, whereas a devotee who not only is expert in devotional service but who also preaches the cult of bhakti and defeats all kinds of agnostics is called a gosthyanandi.