Wer der Welt entsagt, muß alle Menschen lieben, denn er entsagt auch ihrer Welt. Er beginnt daher, das wahre menschliche Wesen zu ahnen, das nicht anders als geliebt werden kann, vorausgesetzt, daß man ihm ebenbürtig ist.
Anyone who renounces the world must love all men, for he renounces their world too. He thus begins to have some inkling of the true nature of man, which cannot but be loved, always assuming that one is its peer. [Kaiser/Wilkins]
Whoever renounces the world must love humanity, because he is also renouncing their world. Accordingly, he will begin to have a true sense of human nature, which is incapable of anything but being loved -- assuming, that is, that one is on the same footing as it. [Hofmann]
This says that, when someone renounces the world, he or she doesn't give up only his or her world, but the world of humanity as such. This means that the renunciate doesn't retire to a private world, but either gives up any world, or enters into some higher, superhuman world that is not his or hers. Perhaps this means that renouncing the world is not just giving it up, but giving it as a gift; which would mean in turn that the renunciate doesn't turn from the world as a worthless mistake or an illusion. The world would therefore be renounced even as it is acknowledged to be a true value.
It could be that Kafka means the world may depend in part on renunciates, because they contend with the whole world as such and so bring the whole world into experience. The result of this is a better understanding of what humanity is, presumably by seeing how humanity understands the world, which can only be a concept, being too big and old to fit into human experience.
Being a peer of mankind -- and ebenbürtig can mean evenly matched as well, so this equivalence is not necessarily a peaceful one! -- is necessary if one is to be in a condition of loving mankind. That means that the renunciate, who loves humanity, must continue to be human or at least at a human level. The renunciation doesn't make him an angel, it makes him or requires of him that he be a lover of mankind. Mankind can only be loved, but only by a peer, which might not mean another human, it need only mean someone at a human level. Loving mankind doesn't make you human, but only a peer of humanity, as it makes it possible for you to renounce the world. Kafka seems to be saying that every lover of mankind must renounce the world. I don't think he means that you have to renounce mankind in order to love it, but that it if you love it, you must renounce it in order to be its peer, which is necessary in order to go on loving it.
Why can't you hate mankind? Is it because hating mankind is still very human, while loving mankind seems to be superhuman? Why can't you be indifferent to mankind? Is it because that indifference is only a kind of subhumanity, which puts you below the level of mankind? There might be overtones here of the previous aphorism about cheating, too much, too little, in the middle.