Spinoza's Ethics: I.D6: God

This definition builds on I.D3 of substance as in itself. The definition of God also expands the role of attribute defined in I.D4. God is characterized as substance made up of infinite attributes. Significantly, the emphasis on attributes places God in a “view from somewhere” categorization. Without the perceiving intellect, God, as defined, does not exist. Without the intellect, God is simply substance without the “as if” or tanquam of infinite attributes. Thus, understanding God involves understanding attributes. Spinoza is defining a relationship between God and the perceiving intellect as real through connecting the expressiveness of the attribute to the perceptiveness of the intellect. 

The explanation (a first so far) highlights this. To be infinite in its own kind (echoing I.D2 where "finite in its own kind" was defined) is to be limited to one attribute only - that of itself. Spinoza states that this expressiveness of God which conveys God’s eternal and infinite essence cannot occur with something that is infinite only in its own kind. To be infinite only in its own kind is to suffer a negation, to be able to be conceived of as not existing and thus to such a degree is incompatible with the essence of substance. Thus, God is substance made expressive through infinite attributes. As God, substance as Knowing and Being also becomes substance as Thinking (and Extending) - a Spinozist variant of the Word becoming incarnate.

Per Deum intelligo ens absolute infinitum hoc est substantiam constantem infinitis attributis quorum unumquodque aeternam et infinitam essentiam exprimit.

Translated as,

By God I understand a being absolutely infinite, that is a substance consisting of an infinity of attributes whose each and every one expresses eternal and infinite essence.

Explicatio: Dico absolute infinitum, non autem in suo genere; quicquid enim in suo genere tantum infinitum est, infinita de eo attributa negare possumus; quod autem absolute infinitum est, ad eius essentiam pertinet quicquid essentiam exprimit et negationem nullam involvit.

Translated as,

Explanation. I say absolutely infinite, however not infinite in its own kind; for whatever is infinite in its own kind only, we are able to deny infinite attributes concerning it; however that which is absolutely infinite, whatever pertains to its essence expresses its essence and involves no negation.