The idea here is to pose a question to a situation, and possibly come up with an answer or two, that could be used as an adventure hook.

The hook:

A warp storm clears after raging for thousands of years. This opens a path to a lost planet/system of planets. The people are fully loyal to the Emperor but have been cut off since the time either just previously to or just after the Horus Heresy. They have no concept of the modern Imperium but have been and are utterly loyal to the pre-heresy Imperial truths of science over religion.

The technology level would be anywhere from modern to near forge world.

How would the modern Imperium react to their existence, and would they or could they even be converted to the Imperial Creed?

I would imagine the Ecclesiarchy and Inquisition would both be quick to label them Hereticus Extremus.

Loyalty to the Emperor after 11,000 years? Surely their faith would have wavered or altered somewhat in that period.

Adeptus Missionaria would try to convert them to the faith of the Emperor-as-God. If that failed, they would be converted by force. Mechanicus would swoop in as well, trying to establish contact with any of the Omnissiah's followers and/or label any non-STC innovations as heretek.

ItsUncertainWho said:

Planetwise genocide most probably. At least that is what happened to the Adranti during the Angevin Crusade in the Calyx Expanse...

Though one could again say 'it depends' (as most of the time regarding 40K). As it matters how heretical their techonology is (or how probable it is to find an undamaged STC) or how easy it would to convert them to the Imperial Creed if it is at all possible (being mostly atheistic and all).