2 Timothy 3:16-17
Very true, good quote. But you perhaps think that this verse means all you need is Scripture --- Scripture and nothing else --- but that's not what it says.
They also had the example and the oral preaching of the Apostles, and the actual practice of worship and governance of the first (apostle-founded) Churches, which sustained the whole Church for several generations before the whole NT was written, collected and and canonized.
Otherwise, you'd be an a position of saying there was "no Church" before the NT Scriptures were completed, which would leave you with "no Church" until about 90-95 AD, when the epistles of John were completed. This is obviously untenable. There were hundreds of local churches scattered all over the Middle East, Turkey, Greece, northern Africa, etc --- founded on the preaching, teaching, and example of first and second generation missionaries, in the absence of the NT canon.
St. John says, several times, that a whole lot more was said and done, than got written down; and St. Paul says to be guided by the preaching and by his example and that of the other apostles and evangelists in their midst.