I want to know the answer to the following question:
What year did the ruling "only a cardinal was eligible to become pope" come in effect?
All the background I'm provided with is that "There is disagreement about the origin of the term, but general consensus that "cardinalis" from the word cardo (meaning 'pivot' or 'hinge') was first used in late antiquity to designate a bishop or priest who was incorporated into a church for which he had not originally been ordained.". Is that enough to answer the question?