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.
After all, Krishna Consciousness philosophy is as old as 120 million years at the least. So nothing can be compared with our philosophy either in the matter of antiquity, philosophy, ethics, science, morality, etc., all in correct vision and approved by great stalwart acaryas. So far others are concerned, they cannot be compared even. For example, if Lord Jesus Christ said "Thou shalt not kill", or "Thou shalt do no murder" to the people, it does not reflect very good social structure of the audience. Our philosophy is above all these things. Just like we prescribe to our students no illicit sex-life, no meat-eating, no intoxication, no gambling, but they are not ends in themselves. The real end is how to serve Krishna and sacrifice everything for Him. And to learn this transcendental art we have got so many volumes of books. So the summary is that instead of diverting our attention to read such unauthorized books, better pay our attention to more authorized Vaisnava literature. These scriptures of the Buddhists and the Christians may be the words of God, but still the are not always applicable to us. It is just like a king may give some rules and regulations for some criminals in prison; but for the good citizens out of the prison these rules are not necessarily applicable. So these Christian and Buddhist scriptures were delivered for a different class of men, and we needn't spend our time in studying their doctrines. You should read our own books over and over again and as far as possible do not try to enter into controversy. We do not concern ourselves with any other religion. Our religion is to become the servant of the servant of the servant of Krishna (CC Madhya 13.80).