[NOTE -- The Necessity of Atheism was published by Shelley in 1811. In 1813 he printed a revised and expanded version of it as one of the notes to his poem Queen Mab. The revised and expanded version is the one here reprinted.]
1st, The evidence of the senses. If the Deity should appear to us, if he should convince our senses of his existence, this revelation would necessarily command belief. Those to whom the Deity has thus appeared have the strongest possible conviction of his existence. But the God of Theologians is incapable of local visibility.
ANSWER: The God of the Bible has made Himself known. He has provided miracles as proof.
2d, Reason. It is urged that man knows that whatever is must either have had a beginning, or have existed from all eternity, he also knows that whatever is not eternal must have had a cause. When this reasoning is applied to the universe, it is necessary to prove that it was created: until that is clearly demonstrated we may reasonably suppose that it has endured from all eternity. We must prove design before we can infer a designer. The only idea which we can form of causation is derivable from the constant conjunction of objects, and the consequent inference of one from the other. In a base where two propositions are diametrically opposite, the mind believes that which is least incomprehensible; -- it is easier to suppose that the universe has existed from all eternity than to conceive a being beyond its limits capable of creating it: if the mind sinks beneath the weight of one, is it an alleviation to increase the intolerability of the burthen?
ANSWER: "FIRST CAUSE" can be inferred by the logical necessity of time limitation, for if time has an infinite past, then it would have taken an infinite amount of time to get to the present in which case the present never exist. But since the present does exist, time must be limited. The other possibility is time itself being repetitive, but again this would violate natural law, which is somewhat the point. There is logically something beyond natural law - which Christians refer to as the super-natural.
The other argument, which is founded on a Man's knowledge of his own existence, stands thus. A man knows not only that he now is, but that once he was not; consequently there must have been a cause. But our idea of causation is alone derivable from the constant conjunction of objects and the consequent Inference of one from the other; and, reasoning experimentally, we can only infer from effects caused adequate to those effects. But there certainly is a generative power which is effected by certain instruments: we cannot prove that it is inherent in these instruments" nor is the contrary hypothesis capable of demonstration: we admit that the generative power is incomprehensible; but to suppose that the same effect is produced by an eternal, omniscient, omnipotent being leaves the cause in the same obscurity, but renders it more incomprehensible.
ANSWER: Though the generative power is incomprehensible to you, that doesn't mean that it is imcomprehensible to me or even that it is incomprehensible to any open-minded individual.
3d, Testimony. It is required that testimony should not be contrary to reason. The testimony that the Deity convinces the senses of men of his existence can only be admitted by us, if our mind considers it less probable, that these men should have been deceived than that the Deity should have appeared to them. Our reason can never admit the testimony of men, who not only declare that they were eye-witnesses of miracles, but that the Deity was irrational; for he commanded that he should be believed, he proposed the highest rewards for, faith, eternal punishments for disbelief. We can only command voluntary actions; belief is not an act of volition; the mind is ever passive, or involuntarily active; from this it is evident that we have no sufficient testimony, or rather that testimony is insufficient to prove the being of a God. It has been before shown that it cannot be deduced from reason. They alone, then, who have been convinced by the evidence of the senses can believe it.
ANSWER: If you were God, how would you prove your existence? You would have to do miracles, even as you admit. But if miracles are done, they occur at points in time, after which they are history. You logically cannot discount historical accounts of miracles merely on the basis that they are improbable. For then you would logically disallow any method for God to prove himself to you, except if he were to do a miracle to you personally. But if miracles were constantly being performed to all of us, they would no longer be considered miracles, but just common natural occurances. Belief on the basis of other people's testimony is very common in life. It is used in schools by teachers; it is used in the courtroom by jurys; etc.
Furthermore, with respect to "faith". You are using an unBiblical definition of it. For Biblical faith is an act of the will. But it is not "blind faith". Sufficient evidence is a necessary basis for it, as in Rom 1:20 an other places declare.
Hence it is evident that, having no proofs from either of the three sources of conviction, the mind cannot believe the existence of a creative God: it is also evident that, as belief is a passion of the mind, no degree of criminality is attachable to disbelief; and that they only are reprehensible who neglect to remove the false medium through which their mind views any subject of discussion. Every reflecting mind must acknowledge that there is no proof of the existence of a Deity.
ANSWER: The Bible agrees with you that if there is not sufficient evidence of God's existence, then people would have excuse and would not be held responsible, but there is. Perhaps you should "reflect" a little bit harder.