Depends on the thing set apart. If you're talking about yourself (assuming you're male), you could say segregatus. If the subject is female, it would be segregata, and if it's an undetermined object, segregatum. Nouns all have gender, though, so if it's referring to e.g. a book, name, or house, it would all be different. Same with whether you're referring to one thing or many.
Doing something for another typically gives the dative case, which for Deus is a simple Deo. More clarification on what's being segregating will yield you the full phrase.
The "in evangelium" here is translating εἰς εὐαγγέλιον; εἰς + acc. can denote purpose in obtaining that object, which works less well in Latin and especially with segregatus.