The characters combine literally to mean “difficult as if climbing to heaven.” The imagery is potent and universally understood: a task so immense it's comparable to an ordinary person trying to reach the heavens.
The power of “难如登天” comes from ancient Chinese cosmology. Heaven (天, tiān) was not just the sky; it was a sacred, unreachable realm, the abode of the Jade Emperor and other deities. For a mortal to ascend there was the ultimate impossibility, a concept deeply embedded in folklore and literature like *Journey to the West*. A useful Western comparison is the phrase “a Herculean task,” which references the near-impossible Twelve Labors of Hercules in Greek mythology. Both expressions invoke a mythological or superhuman scale of difficulty. However, “难如登天” has a more universal and cosmological feel. While Hercules's tasks were specific feats of strength, “ascending to heaven” is a fundamental impossibility of physics and existence in the ancient worldview. It highlights a barrier not just of physical strength but of cosmic order, making the idiom feel incredibly profound. It reflects a worldview where there is a clear and unbridgeable gap between the mortal and celestial realms.
“难如登天” is a fairly formal and literary idiom, but it is widely understood and can be used in spoken Chinese to add emphasis or a dramatic flair.