I happen to be a huge fan of Kafkas writing and count The Trial as my favorite book of all time. I looked it up, cause it sounded weirdly uncharacteristic of him. I've read a lot of Kafka myself and some of his novels deal extensively with the overbearing horror of a bureaucratic state or the universe.
First all, it's actually a mistranslation. He didn't say "back the world", but instead said something closer to "submit to the world" or even closer "make yourself second to the world.". The Trial was written before and The Castle after this quote, so it's easy to assume this quote is somehow consistent with his writings. The quote itself stems from the Zürau aphorisms, a number of meditations he wrote. Interestingly the next aphorism is "One must not cheat anyone, not even the world of its victory.". However, much more interesting is the one immediatly after, in which Kafka states his opinion, that only a spiritual world exists and that our perception of it is rooted in evil. Kafka was jewish and very strongly so. His writings on theology are perfectly consistent with everything I know of jewish theology. Unless I missed something, I believe the world is not meant to refer to society, but to the spiritual world, i.e. god and religion.
To conclude, the quote is not meant to be interpreted as a call towards submission to society or a call towards selflessness. It is instead meant to state that one is to submit themself to god and the spiritual world. To finally give my opinion, I disagree with it.