If Scripture Is Sufficient, Why the Need for Church Authority?

I just stumbled on this wonderful passage from St. Vincent of Lérins:

"I have often then inquired earnestly and attentively of very many men eminent for sanctity and learning, how and by what sure and so to speak universal rule I may be able to distinguish the truth of catholic faith from the falsehood of heretical pravity; and I have always, and in almost every instance, received an answer to this effect: That whether I or any one else would wish to detect the frauds and avoid the snares of heretics as they rise, and to continue sound and complete in the catholic faith, we must, the Lord helping, fortify our own believe in two ways; first, by the authority of the Divine Law, and then, by the Tradition of the catholic Church."

It is interesting to me that finding the one Church is such a pressing topic on St. Vincent's mind back in 434 A.D., when this was written. How much more difficult this is in today's world with such a wide array of variation of faiths which carry the label "Christian" on them. St. Vincent, after much asking of men more learned and holy than him, consistently heard two simple answers:

1. The Holy Scriptures, or "the authority of the Divine Law"

2. Tradition - that which has been passed down

The majority of Protestants would quickly react to the "Tradition" he mentions. Evidently, that was also and issue in St. Vincent's day, because he immediately continues thus:

"But, here some one perhaps will ask, Since the canon of Scripture is complete, and sufficient of itself for everything, and more than sufficient, what need is there to join with it the authority of the Church's interpretation? For this reason, —because, owing to the depth of Holy Scripture, all do not accept it in one and the same sense, but one understands its words in one way, another in another; so that it seems to be capable of as man interpretations as there are interpreters ... Moreover, in the catholic Church itself, all possible care must be taken, that we hold that faith which has been believed everywhere, always, by all. For that is truly and in the strictest sense "catholic", which, as the name itself and the reason of the thing declare, comprehends all universally. This rule we shall observe if we follow universality, antiquity, consent. We shall follow universality if we confess that one faith to be true, which the whole Church throughout the world confesses; antiquity, if we in no wise depart from those interpretations which it is manifest were notoriously held by our holy ancestors and fathers; consent, in like manner, if in antiquity itself we adhere to the consentient [same opinion of, agreement in] definitions and determinations of all, or at the least of almost all priests and doctors."

—St. Vincent of Lérins, A Commonitory ("An aid to memory"), chapter II.