Is it some kind of virus or antivirus?

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It is a virus... but it's a computer-friendly countervirus. It's Arunmor's answer to Revelation, which Faith will liberate from any system it is run on.

I got Revelation from that Arunmor's Server. There was also a file called Revelation_Tracer (which I also downloaded). How does that help us during missions? or it doesn't have any real function?

What do you mean when you say "will liberate from any system it is run on"? and I get it at Arunmor's File Servers?

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Revelation_Tracer is part of Arunmor's second story mission... and a better version of it in their last. I recommned you do those mission to get a full account of what it is.

By liberate, Revelation takes a small amout of time to take over a system. Running faith on a system that is being taken over by Revelation will basically kill it

To get Faith, the only way you can do that on PC version is play the Arunmor story to its conclusion, so I assume it's the same for you.