"Jesus did many other things as well. If every one of them were written down, I suppose that even the whole world would not have room for the books that would be written."
What a shame that the Gospel of John ends like this. How wonderful would it have been to have a detailed biography of Jesus's life? How marvelous would it have been if Jesus himself had written a book? Actually, He did.
The Bible is the written word of God as transcribed through his prophets and apostles. Both Old and New Testament have been written under the influence of the Holy Spirit. Jesus did leave us instructions for future generations.
But why only these few things and not others? Christ has given us everything that is necessary and sufficient for our salvation, nothing more and nothing less. Everything we ever needed to know about God is contained within Scripture. Yet there is so much we do not know, but I suppose if God had revealed to us the fullness of the truth, the whole world would not have room for all the books that would be written. How vast and deep is the knowledge and wisdom of God!
So all that we have is all that we need. Everything else is extraneous to the gospel. However, whatever is not necessary for salvation can be done in the name of ministry. Anything and everything should be done with salvation in mind and the kingdom at hand.