One of the marked features of the reign of Domitian is attention which he devoted to the restoration of the national cultus. In this respect his policy was the same as that of Augustus; and, like him, he looked on the Imperial cultus as part of the national religion. He himself delighted to be identified with Jupiter, and to be idolised as the Divine Providence in human form; and it is recorded that Caligula, Domitian, and Diocletian were the three emperors who delighted to be styled dominus et deus. Though a certain element of individual caprice is discernible in the extent to which Domitian pushed the personal reference, yet the policy is not peculiar to him, but was a fixed and highly important part of the general Imperial policy, which treated religion as a part of the machinery of government. In this point of view, refusal to comply with the prescribed forms of respect to the Emperor was a refusal to be a member of the Roman unity, and constituted disloyalty and treason. As we have already seen, Pliny found the procedure already established that a charge of Christianity should be tested by calling on the accused to perform the ceremonies of loyal service and worship to the Emperor. Christianity was disloyalty; and, conversely, the mere rendering of the duties of loyalty disproved Christianity.
The scanty evidence which we have found, therefore, seems to point to the view that Christianity was, under Domitian, treated as treasonable. This implies that the trials now assumed a new form. Individual Christians were no longer proved guilty of acts which showed hostility to the existing system of society; but the whole principles and constitution of the sect were condemned as hostile to the established order, and mere membership of the sect, if persisted in, was reckoned as treasonable. The Christians, as a body were outlaws, and were treated as such as soon as their adherence to the sect was recognised; and the trial was conducted only with a view of establishing the fact that the accused persons were Christians. Such was the cognito which Pliny applied as a regular process to the first cases that were brought before him.
from page 275-276, The Church in the Roman Empire