A few days ago HBO/Cinemax released a big list of all their upcoming projects for the next year - Outcast was on the list, but unlike most of the other shows which were given either exact dates or vague estimates (Early 2018, etc.), it didn't get anything. Which is extremely odd considering that it already aired in its entirety months ago in some countries.
Cinemax is rebooting itself next year with a whole new lineup, but they also have Mike Judge's new animated comedy scheduled for September so I don't know why they didn't just go ahead and announce the date for Outcast too.
Outcast was meant to be Cinemax's next big show, appealing to mainstream audiences in the same way that Strike Back and Banshee did, but instead it totally cratered. The show's future was always uncertain due to that fact, but the way they handled the (ordered before season one even premiered) second season is totally bizarre.
Unless the show is a hit internationally, which it might be considering that they actually bothered to air it internationally, I can't imagine we'll get a third season.