What I truly want you to understand about Spinoza is his Double Aspect Theory and criticism of Descartes. You need to know that there is only one eternal infinite substance, and that this is the Absolute. What dualists like Descartes call substances, cannot be substances, since they cannot be conceived alone, but only through something else. This dualism of Descartes left him caught in solipsism. Solipsism is the view that all one can know is their own ideas, and not if anything real corresponds to them.
Truth is a matter of correspondence to reality.
True ideas, or true statements match or correspond to a true state of affairs. In order to verify this correspondence it is necessary that the idea in the subject be compared to the object. But Descartes' separate and distinct substances of mind and body do not permit this kind of verification. Descartes is caught in his mind, thinking about the doubt, and he cannot be certain if any of his ideas correspond to anything real in the external world of the bodily mode. Descartes is caught in solipsism.