The Loeb translation by Richard M. Gummere of Seneca's Epistula XLI, "On the God Within Us":
Non sunt ad caelum elevandae manus nec exorandus aedituus ut nos ad aurem simulacri, quasi magis exaudiri possimus, admittat; prope est a te deus, tecum est, intus est. Ita dico, Lucili: sacer intra nos spiritus sedet, malorum bonorumque nostrorum observator et custos. Hic prout a nobis tractatus est, ita nos ipse tractat. Bonus vero vir sine deo nemo est; an potest aliquis supra fortunam nisi ab illo adiutus exurgere? Ille dat consilia magnifica et erecta. In unoquoque virorum bonorum:
Quis deus incertum est, habitat deus.
renders deus as "God" in the main text but "a god" in the quotation:
We do not need to uplift our hands toward heaven, or to beg the keeper of a temple to let us approach his idol's ear, as if in this way our prayers were more likely to be heard. God is near you, he is with you, he is within you. This is what I mean, Lucilius: a holy spirit indwells within us, one who marks our good and bad deeds, and is our guardian. As we treat this spirit, so are we treated by it. Indeed, no man can be good without the help of God. Can one rise superior to fortune unless God helps him to rise? He it is that gives noble and upright counsel. In each good man:
A god doth dwell, but what god know we not. [Vergil, Aeneid, viii.352]
Is the translation to "God" an imposition of Judeo-Christian monotheism onto the Roman pagan worldview, or is Seneca really invoking a notion of a single "God", common to everyone, which he expects Lucilius to understand? Since Latin lacks definite and indefinite articles, I figure you have to refer to the cultural context, but if I'm wrong about that, please let me know.