Srimad Bhagavatam Canto 04, Chapter 19, Text 37

SB 4.19.37

bhavan paritratum ihavatirno
dharmam jananam samayanurupam
venapacarad avaluptam adya
tad-dehato visnu-kalasi vainya
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O King Prthu, son of Vena, you are the part-and-parcel expansion of Lord Visnu. Due to the mischievous activities of King Vena, religious principles were almost lost. At that opportune moment you descended as the incarnation of Lord Visnu. Indeed, for the protection of religious principles you have appeared from the body of King Vena.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The way in which Lord Visnu kills the demons and protects the faithful is mentioned in Bhagavad-gita (4.8):
paritranaya sadhunam
vinasaya ca duskrtam
sambhavami yuge yuge
“In order to deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear, millennium after millennium.”
In two hands Lord Visnu always carries a club and a cakra to kill demons, and in His other two hands He holds a conchshell and a lotus to give protection to His devotees. When His incarnation is present on this planet or in this universe, the Lord kills the demons and protects His devotees simultaneously. Sometimes Lord Visnu appears in His person as Lord Krsna or Lord Rama. All of these appearances are mentioned in the sastras. Sometimes He appears as a saktyavesa-avatara like Lord Buddha. As explained before, these saktyavesa-avataras are incarnations of Visnu’s power invested in a living entity. Living entities are also part and parcel of Lord Visnu, but they are not as powerful; therefore when a living entity descends as an incarnation of Visnu, he is especially empowered by the Lord.
When King Prthu is described as an incarnation of Lord Visnu, it should be understood that he is a saktyavesa-avatara, part and parcel of Lord Visnu, and is specifically empowered by Him. Any living being acting as the incarnation of Lord Visnu is thus empowered by Lord Visnu to preach the bhakti cult. Such a person can act like Lord Visnu and defeat demons by arguments and preach the bhakti cult exactly according to the principles of sastra. As indicated in Bhagavad-gita, whenever we find someone extraordinary preaching the bhakti cult, we should know that he is especially empowered by Lord Visnu, or Lord Krsna. As confirmed in Caitanya-caritamrta (Antya 7.11), krsna-sakti vina nahe tara pravartana: one cannot explain the glories of the holy name of the Lord without being specifically empowered by Him. If one criticizes or finds fault with such an empowered personality, one is to be considered an offender against Lord Visnu and is punishable. Even though such offenders may dress as Vaisnavas with false tilaka and mala, they are never forgiven by the Lord if they offend a pure Vaisnava. There are many instances of this in the sastras.