Here is an extract from F.P.Kenrick's Theologia Moralis (1860):
40. Negative infidelity is the term used in respect of those who have never heard anything about the Faith. Positive infidelity is the term used in respect of those who have had the Faith preached to them and have rejected it. Privative infidelity is the term used in respect of whose who have had the Faith adequately preached to them but have not accepted it. Negative infidelity is free from sin, since it is not voluntary.... Indeed, Christ himself said, "If I had not come and spoken to them, they would not have sinned, but now they have no excuse for their sin" [Jn. 15.22]. Privative infidelity is a mortal sin, for "he who does not believe will be condemned" [Mk. 16.16]; "but he who does not believe already stands judged" [Jn. 3.18]. When the Faith has been preached to a man, and the arguments and documents pertaining to it have been presented to him, he is indeed bound to give his obedience to the divine truth, and he cannot suspend his assent to it without doing violence to the majesty of God. We must take the view that a man sins much more gravely if he rejects the truth and embraces a contrary error. The fact that this is an error of the intellect does not remove the guilt, for it is a fault of the will that the intellect does not thoroughly weigh the evidence by which revelation is proven and fails to embrace it....
41. Paganism, Judaism, Mohammedanism and heresy are types of infidelity. Paganism is the name given to the worship of many gods and idols.... Atheism, which entirely denies the existence of a Divine Power, is to be considered in a similar light, as is pantheism, which claims that all things are God. Judaism is the worship of the one God in accordance with the rites formerly laid down by him, but which were abrogated when Christ came. Mohammedanism is the worship of God according to the teachings of Mohammed, who falsely claimed that he was a prophet and mixed together many teachings which he had borrowed from the Gentiles and the Jews. Heresy is the impairment of some part of the Christian faith.
42. Those who have not entered into the Church by Baptism are not at all to be compelled to accept the Faith or to fulfil the duties of Christian men.... Therefore, they are to be induced to enter the Church only by persuasion, encouragement and prayer.... However, as for apostates and heretics "who once accepted the Faith", [St Thomas Aquinas] takes a diferent view: "Such men are to be compelled even physically to fulfil what they have promised, and to hold to what they once accepted". These differ to a degree from other men, who have adhered to the errors of their ancestors. The tribunal of the Inqusition itself took the view that it was proper to deal with these people in a milder manner. Today, however, force is not used anywhere even to compel apostates to keep to what they have promised."
Part of the context in which Kenrick was writing was the belief that it was quite easy to determine that Catholicism was the true faith. This is what Pope Leo XIII said in his encyclical Immortale Dei (1885):
"Now, it cannot be difficult to find out which is the true religion, if only it be sought with an earnest and unbiased mind; for proofs are abundant and striking. We have, for example, the fulfilment of prophecies, miracles in great numbers, the rapid spread of the faith in the midst of enemies and in face of overwhelming obstacles, the witness of the martyrs, and the like. From all these it is evident that the only true religion is the one established by Jesus Christ Himself, and which He committed to His Church to protect and to propagate."
Leo later said this in Libertas (1888):
"And if it be asked which of the many conflicting religions it is necessary to adopt, reason and the natural law unhesitatingly tell us to practice that one which God enjoins, and which men can easily recognize by certain exterior notes, whereby Divine Providence has willed that it should be distinguished, because, in a matter of such moment, the most terrible loss would be the consequence of error."
Some years later, Ludwig Ott wrote in his Fundamentals of Catholic Dogma (1952):
"Negative atheism is inculpable ignorance regarding the existence of God. Positive atheism (materialism, pantheism) directly denies the existence of a supramundane, personal Divine Being. It was condemned by the [First] Vatican Council....
As far as the possibility of atheism is concerned, it cannot be denied that there are atheistic doctrinal systems (materialism, pantheism) and practical atheists, that is, people who live as if there were no God. The possibility, that there are also subjectively convinced theoretical atheists, is founded in the spiritual and moral weakness of man, and on the fact that the proofs of God are not immediately, but only mediately evident. But as the knowledge of God can easily be gained from the contemplation of nature and the life of the soul, it will not be possible permanently to adhere to an honest and positive conviction of the non-existence of God. An inculpable and invincible ignorance regarding the existence of God is not possible for a long time in a normal, grown-up person, in view of the facility of the natural knowledge of God attested in Holy Writ and in Tradition."