The books which were candidates for inclusion were scrutinized carefully to make sure they were genuinely written by the disciples or those extremely close to the disciples (Mark for example.)
The reason it was so important to have works emanating from the disciples' era was that they had first-hand experience of Jesus and were filled with the Spirit. Because these early men foresaw that a period of heresy was about to begin (with people claiming all sorts of things about Christ), they knew the importance of including works only associated with those near Christ.
The more I learn about the formation of the Bible, the more I know it it trustworthy.
Well, St. Luke did not have first-hand knowledge of Jesus. But he did meet (gasp!) Mary.