In his disputation on the simple essence of God, Voetius commends Rada and the Scotists on the question of whether or not Christ ought to be referred to as a composite person (no). I am inclined to agree with Voetius here and take a generally Scotus position on Christology, but I have much to learn here. Because of this, I thought it worthwhile to read Rada on christology. So what follows is the entire section on Rada from Richard Cross's book on christology. I currently understand close to none of it, so I hope to revisit this in the future.
In 1586 Rada published an account of the controversies then current between Thomists and Scotists—predating, of course, Suárez’s decisive intervention in proposing a comprehensive theory of modes to resolve the worries surrounding Scotus’s characterization of the union as an accident. Indeed, Rada’s principal contribution to Christological discussions lies in his formulating a response to the Thomist objection to Scotus’s claim that the union could be a categorial relation. As we have seen, the worry is that a categorial relation is an accident, and no substantial unity can include an accident. It is this difficulty, as we have just seen, that led Suárez to posit that the union is a substantial mode. In response to the objection, Rada makes a distinction between the union considered as an accident and the union considered as accidental:
The union of the humanity with the Word is, taken formally, an accident.…But it is not on that account licit to infer that the union is accidental. For it is one thing for the union to be an accident, and another for it to be an accidental kind of union. For it is not from the fact that it is an accident or a substance that the union is accidental or substantial, but from the fact that the extremes are united accidentally or substantially. Whence the union of clothing with a human being is accidental because clothing does not give substantial esse but accidental [esse] to a human being, namely, being clothed. The union of a substantial form with matter is substantial, since they are united to constitute a substance, that is, a composed [substance]. Likewise, the union of the humanity of the Word is called substantial, not because it is a substance, but because the humanity is united to it according to substance, that is, according to hypostasis which is a substance, by attributing true substantial esse to the Word—namely, the esse of a human being—excluding all information, for from the force of the union the Word is truly a human being, and a substance, and a what (quid), and not of a certain condition (quale) or quantity (quantum), or relation (ad aliquid).
The line of reasoning is very simple. What is united to the Word is a substance, and it is that in virtue of which the Word comes to be a substance of a given kind (a human being). So the type of unity conferred by the relation is substantial, even though the union itself is an accident. On Rada’s reading, the opponents’ view seems to presuppose the self-predication of the forms: in this case, that what explains substantial unity must itself be a substance or a substantial unity. But there is no obvious reason to accept this. And note that, while Rada here construes the human nature’s ‘attributing true substantial esse to the Word’ in an ontologically robust way, the argument in favour of non-accidentality does not need to make this strong assumption. It is uncontroversially true that the Word’s union with the human nature is supposed to explain the Word’s being human, and thus the Word’s being in some sense a human substance or person, whether or not the metaphysics requires ‘attributing true substantial esse to the Word’.
In good Scotist fashion, Rada maintains that the human nature must have its own existence. He repeats some of Scotus’s arguments in favour of this, but adds some more of his own, relevant to contemporaneous debates. In particular, he worries about the explanatory circularity that seems to be involved in the view that the existence of the human nature is the esse of the Word:
The humanity, as an actual entity in the order of nature, is presupposed to the assumption. Therefore it is not constituted in the actual esse of entity by the assumption, and, consequently, neither [is it constituted] by uncreated existence, because, since that is the end term of the assumption, it cannot be presupposed to it.
Equally, it cannot be the case that the nature ‘as naturally apt to exist’ is presupposed to the assumption: those who assert this ‘say something unbelievable, because a thing understood prior to its existence is only in objective potency to existing’. As objectively potential, the nature lacks any actuality, and thus lacks what is required for being the subject of a real relation. Finally, Rada repeats the claim that the communicationR of divine esse would result in the incarnation of the whole Trinity (since what replaces the created esse is something common to all three persons). To the Thomistic reply that union with divine esse is mediated through union to the subsistence of the Word, Rada responds that the existence of the nature is antecedent to its subsistence in the Word, and hence that union with divine esse would be antecedent to union to divine subsistence. This, of course, presupposes Scotus’s nature–esse–subsistence sequence, and thus would not necessarily convince someone of a more Thomistic persuasion.
Immediately after this, Cross moves on to the thought of Bartholomew Mastrius, greatly influenced by Rada here, and so Mastri's comments also ought to be consulted.