There were dozens of supposedly "inspired" books in circulation declared non-canonical by Pope Boniface. That was the whole point of declaring a canon. In a sense, it was already "in place" (it was "in place" from before the beginning of time), but human beings did the discerning by aid of the Holy Spirit, among many more texts than appear in the Bible as we know it.
"Those quotes were written by Christians. The books of the New Testament were already in circulation before the end of the first century. The Holy Spirit decided canon, and the the church simply recognized what was already in place."
Yes. The books of the NT were already in circulation before the end of the 1st century...as were hundreds of other gospels and writings such as the Gospel of Judas, the Gospel of Thomas, etc...
Yes. The Holy Spirit did decide on Canon...through the undivided, pre-denominational Church of faithful. The same Church that has been faithful to all that the Holy Spirit did for 2000 years since Christ. It didn't just fall from the sky or appear out of thin air. God works through the faithful. The same Holy Spirit present when books were being decided did not just come and go as other issues were being decided (at the same moments and same meetings).
They recognized the Truth because they were guided by the Spirit of Truth...again not just about the canon of books.
Why were the most important issues facing the Church decided by the Church, visible, and still are today? Maybe because the Holy Spirit was present? I think so!
Look how the Apostle came together in Acts and found the it was good to both them and the Holy Spirit...how did "they" know that when even St Peter was wrong "alone". Because the Holy Spirit works through the Church, the body of Christ, and not the individual per se (not to be misinterpreted as the individual not having the Holy Spirit, but that the individual has a higher probability of following their own passions and not the Truth than does two or more. But even in todays world it is easy for many influenced by a lifetime of unTruth to miss the Truth of the Holy Spirit and is why we must be humble to the early Church).