How to parse "in eo quod"?
It appears that in eo quod is not a syntagma, but rather that the preposition in goes with manent to form a different syntagma. The syntagma maneō in quōd is used to express to "adhere to something," "abide by something," and "remain in some condition."
Examples from Lewis & Short:
- Pregn., to remain, last, endure, continue in any place or manner: “si in eo manerent, quod convenisset,” would adhere to, abide by that,
Caes. B. G. 1, 36, 5: “in vita,” to remain alive, Cic. Fam. 4, 13, 2:
“in veritate,” to adhere to the truth, id. Clu. 63, 176: “in
condicione,” to fulfil a condition, id. Att. 7, 15, 3: “in sententia,”
to adhere to, id. ib. 9, 2, 1: “in voluntate,” id. Fam. 5, 2, 10: “in
pristina mente,” id. Sest. 27, 58: “in officio,” Hirt. B. G. 8, 47:
“tu modo promissis maneas,” abide by, keep, Verg. A. 2, 160: “in
pactione,” to abide by, Nep. Ages. 2, 4: “an credi posse ullum populum
in ea condicione mansurum?” Liv. 8, 21, 6: “mansit in condicione atque
pacto,” Cic. Verr. 1, 6, 16:
With this knowledge, we can reduce the entire clause to the core statement: ...in eō...manent. In context, this can be understood as "they adhere to that" or "they remain so."
If we expand that core, we get: in eō quod facta sunt...manent. The word quod can only be the predicative nominative governed by facta sunt, giving us: "they adhere to that which they were made" or "they remain as they were made."
If we expand further, we get: in eō quod facta sunt, quantum accēpērunt, manent, with an added parenthetical referring to the same thing as eō and quod. I understand this as: "they adhere to what they were made, as much as they received" or "they remain as they were made, with respect to all they received."
I would translate the entire quote as:
Because when those things that were made remain as they were made with
respect to all they received, like either those things that did not
sin or those that cannot sin, they are both individually good and very
good in the totality.