Die sinnliche Liebe täuscht über die himmlische hinweg; allein könnte sie es nicht, aber da sie das Element der himmlischen Liebe unbewußt in sich hat, kann sie es.
Sensual love deceives one as to the nature of heavenly love; it could not do so alone, but since it unconsciously has the element of heavenly love within it, it can do so. [Kaiser/Wilkins]
Sexual love deceives us as to heavenly love; were it alone, it would not be able to do so, but containing within itself, unknowingly, a germ of heavenly love, it can. [Hofmann]
Sinnliche can mean sensual, but it can also mean simply sensory, which in this case would indicate a love of outward appearance, sense impressions. I don't know that it makes sense to assume that heavenly love is its opposite, especially since Kafka claims the one contains an element of the other. To call this a "germ" implies that the heavenly develops out of the sexual, which is not what Kafka is saying.
How is one deceived? What is the wrong thing that sensual love causes us to think about heavenly love? It seems to involve overextending a comparison between the two.
What is "heavenly love"? Whatever it is, it is not wholly unlike sexual love. It could be that the heavenly attribute attaches to the subject or to the manner of loving, which in this case amount to the same thing, namely, unselfish love. If, on the other hand, it is the object, then this would mean one loves heavenly things. Kafka does not say that heavenly love has an element of sensual love in it; is the formula reversible? Is sensual love de facto selfish?
It could be that Kafka means to draw the distinction between heavenly love, which is not apparent but an object of faith, and sensual love, which is apparent and which attaches to appearances. If sensual love has an element of heavenly love in it, then this would mean it does not respond entirely to appearances. If sensual love appears as heavenly love, this might mean that heavenly love has a way of appearing that can be mistaken for sensual love. If these are true, then heavenly love must not be a matter of faith and sensual love not just a matter of appearances.