P19: God/Nature is eternal, or all God/Nature’s attributes are eternal.

Dem.: For God/Nature (by D6) is substance, which (by P11) necessarily exists, i.e. (by P7), to whose nature it pertains to exist, or (what is the same) from whose definition it follows that he exists; and therefore (by D8), he is eternal.

Next, by God/Nature’s attributes are to be understood what (by D4) expresses an essence of the Divine/Universal substance, i.e., what pertains to substance. The attributes themselves, I say, must involve it itself. But eternity pertains to the nature of substance (as I have already demonstrated from P7). Therefore each of the attributes must involve eternity, and so, they are all eternal, q.e.d.

Schol.: This Proposition is also as clear as possible from the way I have demonstrated God/Nature’s existence (P11). For from that demonstration, I say, it is established that God/Nature’s existence, like his essence, is an eternal truth. And then I have also demonstrated God/Nature’s eternity in another way (Descartes’ Principles IP19), and there is no need to repeat it here.