From this Helvidiusd strives to prove that no one can be called firstborn who has not brothers, as he is called only-begotten who is the only son of his parents. But we thus determine the matter. Every only-begotten is firstborn, not every firstborn is only-begotten. We say not that he is first-begotten whom others follow, but before whom there is no one; (otherwise, supposing there is no firstborn but who has brothers following him, there are then no firstlings due to the priests as long as there are no others begotten;) lest perchance when no birth follows afterward, there should be an only-begotten and not a firstborn.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: cont. Helvid.