The case is NM v. Secretary (Nov. 13); an excerpt (paragraph break added):
We do not think this is an issue as to whether or not an individual in these circumstances is reasonably likely to be discovered on return. The plain fact on the evidence before us is that a genuine apostate, and here we are dealing specifically with conversion from Islam to Christianity, simply would not be able to openly express his change of faith without running a real risk of persecution. The individual would have to keep his faith completely secret; he would have to live a lie; he may be forced to forego contact with others of his faith because of the danger and, significantly, would be constantly looking over his shoulder to avoid discovery in fear of the consequences. In the event it would matter little whether such an individual had family support or not; if discovered the evidence does show that there would be inadequate level of protection available from the Afghan authorities against those who would seek to punish for that conversion.
In our view an apostate could not reasonably be expected to tolerate living in this way in Afghanistan in order to reduce the risk of discovery, and it would be persecutory to expect such an individual to modify his behaviour to that end. It may well be that in some societies solitary and or private worship of another faith may be viable because for example although the background evidence reveals a general intolerance in society toward that belief it does not reach a level where there would be a real risk of ill- treatment on discovery. This is not the case for Afghan converts; there is no evidence that they would be able to conduct themselves in this way. In reaching this conclusion we have borne in mind the Tribunal’s guidance in SZ and JM (Christians – FS Confirmed) (CG)  UKAIT 00082 and HJ (Homosexuality: reasonably tolerating living discreetly) Iran  UKAIT 00044, the latter was approved by the Court of Appeal in XY (Iran) v SSHD  EWCA Civ 911.
For more on a case dealing with a similar question in the U.S., see this post, though in that earlier case there were questions about the applicant’s sincerity that do not appear to be present in this case.