Spinoza, Ethics, De Dei, Propositio 15, Scholium:
Si igitur ipsi ex suo hoc absurdo concludere tamen volunt substantiam extensam debere esse finitam, nihil aliud hercle faciunt quam si quis ex eo quod finxit circulum quadrati proprietates habere, concludit circulum non habere centrum ex quo omnes ad circumferentiam ductæ lineae sunt aequales
I understand the meaning of the sentence but I can not realize the literal meaning of the bolded part because of faciunt and si.
In short Spinoza already explains that some authors conclude from an absurd proposition (A) that the extended substance must only be finite. He demonstrates absurdity of A and hence he deduced that the finiteness of the extended substance is not necessary. Here he says that if those authors persist in believing in A and conclude from it that the extended substance must be finite, they are like one who supposed a circle to have the properties of a square, and then concludes from this that the distance of the points on circumference from the centre of the circle is not equal.