From time immemorial the human race has acknowledged some form of higher presence or being than themselves. Clans, tribes or individuals all had their own idea. Some believe in a particular God, personalized or not. Some people follow nature. Others ascribe to the moon, the sun, the stars, or other things unseen.
There is no such statement in the revealed scriptures [sastras] that Visnu/Krsna [God] reveals His teachings in many places simultaneously. When it is mentioned that religion is taught according to time, place and circumstances, then that is referring to bona-fide teachers of eternal religion like Sri Ramanujacarya and Sri Madhvacarya and not upstarts like Mohammed, Jesus or St. Paul.
Preaching means fighting - fighting with illusion and misconception. Our weapons are philosophical arguments backed by sastra and the opinions of previous acaryas. Preaching means speaking the truth and the nature of truth is that it wants to be found and it wants to fight with ignorance and misconception.
Wasn't there perhaps sincerity in some of these religions at the time when they emerged? God is omnipresent and, after all, a man in the desert who had some desire to know truth would experience an aspect of God's beauty, power, love etc. He could feel that to be God and not yet know about Sri Krishna, due to his unfortunate karma of not being near a pure devotee. Does it mean that he has no realization just because he doesn't yet know about Krishna?
During a recent visit to Europe I had some informal discussions about religious conceptions with other Gaudiya Vaisnavas and I was surprised to hear some devotees speak about such groups as the Sufis, Whirling Dervishes, Jews, Christians, Catholics, Orthodox Christians and Muslims as being deeply 'surrendered' souls. Of course, generally speaking this may be true – but surrendered to what?
There is a misconception that the Krsna consciousness movement represents Hinduism. Krsna consciousness is in no way a faith or religion that seeks to defeat other faiths or religions. Rather, it is an essential cultural movement for the entire human society and does not consider any particular sectarian faith. This cultural movement is especially meant to educate people in how they can love God.