Please help. with a philosophy question. hume

Im writing hume rejects ontological argument. the dialogue is in dialogues concerning natural religion PART IX. there is 4 parts. i have the original text by him. and i am confused on two of the last parts. the first two:

1.cant prove cause and efffect a priori. (need experience)
2. ok to imagine a god which exists and does not exist. (doesn not imply a contradiction)

now the last two parts are in these two paragraphs. i just need an explanation in laymans terms. im lost on the universe necessarily existing.

But further, why may not the material universe be the necessarily existent being, according to this pretended explication of necessity? We dare not affirm that we know all the qualities of matter; and for aught we can determine, it may contain some qualities, which, were they known, would make its non- existence appear as great a contradiction as that twice two is five. I find only one argument employed to prove, that the material world is not the necessarily existent Being: and this argument is derived from the contingency both of the matter and the form of the world. "Any particle of matter," it is said, 7 "may be conceived to be annihilated; and any form may be conceived to be altered. Such an annihilation or alteration, therefore, is not impossible." But it seems a great partiality not to perceive, that the same argument extends equally to the Deity, so far as we have any conception of him; and that the mind can at least imagine him to be non-existent, or his attributes to be altered. It must be some unknown, inconceivable qualities, which can make his non-existence appear impossible, or his attributes unalterable: and no reason can be assigned, why these qualities may not belong to matter. As they are altogether unknown and inconceivable, they can never be proved incompatible with it.

Though the reasonings which you have urged, Cleanthes, may well excuse me, said Philo, from starting any further difficulties, yet I cannot forbear insisting still upon another topic. It is observed by arithmeticians, that the products of 9, compose always either 9, or some lesser product of 9, if you add together all the characters of which any of the former products is composed. Thus, of 18, 27, 36, which are products of 9, you make 9 by adding 1 to 8, 2 to 7, 3 to 6. Thus, 369 is a product also of 9; and if you add 3, 6, and 9, you make 18, a lesser product of 9. 8 To a superficial observer, so wonderful a regularity may be admired as the effect either of chance or design: but a skilful algebraist immediately concludes it to be the work of necessity, and demonstrates, that it must forever result from the nature of these numbers. Is it not probable, I ask, that the whole economy of the universe is conducted by a like necessity, though no human algebra can furnish a key which solves the difficulty? And instead of admiring the order of natural beings, may it not happen, that, could we penetrate into the intimate nature of bodies, we should clearly see why it was absolutely impossible they could ever admit of any other disposition? So dangerous is it to introduce this idea of necessity into the present question! and so naturally does it afford an inference directly opposite to the religious hypothesis!