Stasi
I have seen articles (which used words like "suspected," "presumed," "possibly," "informant who chooses not to be named," etc.) which tried to tie her to the Stasi, but my own hunch is that those who tried unsuccessfully to recruit her, are now trying to defame her.
Not that I (or any of us) have full information about activities which were, by definition, covert and included a lot of convoluted counter-counter-counter agentry. However this article (LINK) is interesting and as believable as any.
Merkel is not a far-left anti-Christian. She is a center-Right (CDU, conservative for Germany) lifelong and still-faithful Lutheran. She's made some horrible --- truly horrible --- policy mistakes, but she at least is now on record saying the "Hajji" cadres will not be staying permanently.