My favorite feline friend ever is being aggressive?

the problem would have started when the family bought the new dog into the home,the cat saw the home as his,and his only,then along comes this huge dog,,cats hate dogs at the best of times,but one that comes into the cats territory is a really big no no,the cat trusted you and loved you as you gave it a lovely home,now you took this away from him,everytime hes in the home,he feels on edge and ready to attack should the dog come near,this is why the cat changed its behaviour,its not helped now by the fact you have moved out and for whatever reason have not took the cat with you,the only way to get back the good relationship you once had with the cat,is to take it back to where you live now,win its trust again by showing you have no dog there,in time the cat will return to its good old self,goodluck with this