I don’t understand. If not for ruling on constitutionality, thus being an equal branch of government, why would the Founders have established it?
If you read the Constitution (I read it to my 7th graders for over 25 years), you’ll see that the Constitution is silent on the subject. In Marbury v Madison the court declared an act of Congress Unconstitutional. The Court took on this power for itself—it is not actually in the Constitution. Whether or not this was or is a good thing is a subject for debate. Congress does have the power (and this power is in the Constitution) to limit the Supreme Court’s jurisdiction. It has never chosen to do so.