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.
Add to this, that in tracing an eternal succession of objects, it seems absurd to inquire for a general cause or first author.

How can any thing, that exists from eternity, have a cause, since that relation implies a priority in time, and a beginning of existence?
In such a chain, too, or succession of objects, each part is caused by that which preceded it, and causes that which succeeds it.

Where then is the difficulty?
But the whole, you say, wants a cause.

I answer, that the uniting of these parts into a whole, like the uniting of several distinct countries into one kingdom, or several distinct members into one body, is performed merely by an arbitrary act of the mind, and has no influence on the nature of things.