Regarding your question about Lord Jesus Christ, we accept him as saktyavesa avatara. Lord Buddha is in the same category also. Lord Buddha is mentioned specifically in Srimad-Bhagavatam as incarnation of Godhead, and yet Vaisnavas do not accept his philosophy, which is classified as atheism. Similarly, even if we accept Lord Jesus Christ as saktyavesa avatara, it doesn’t mean that we have to accept his philosophy. But we have all respects for him without fail. Regarding books like Aquarian Gospel or even the Testiments, we cannot accept them as authorities because sometimes it is learnt that the words are not actually spoken by Christ, but they are so set up by the devotees. For example, in the Ten Commandments it is clearly stated “Thou shalt not kill,” but some Bishop in Boston has changed it to “Thou shalt do no murder.” This means the Bishop wants to keep hold for animal slaughter. So don’t bother about all these literatures. We have all respect for these great preachers, but we do not require to study books save and accept for some reference. We must push on our philosophy how to love God. Our process is simple. We have got volumes of books also, so it is better for us to mind our own business than to divert our attention in the studies of other books. This was definitely forbidden by Lord Caitanya.
From Srila Prabhupada’s letter to Hansadutta — Tittenhurst, 2 November, 1969