'Just' is singular, 'unjust' plural. There is no article in either case. It is not 'the just' par excellence, as Acts 3.14.
The article being left out, it is characteristic, in contrast with 'in flesh.' Both flesh and spirit are the manner and character of what is predicated of Christ. We could say 'present in spirit,' 'fervent in spirit,' because it is characteristic: but 'made alive in spirit' conveys to the English mind the idea of an accomplished fact. It cannot be simply characteristic. In Greek, on the other hand, although conveying a fact, it has a characteristic significance. The sense given here is right.