Second, the verse from John’s Gospel tells us only that the Bible was composed so we can be helped to believe Jesus is the Messiah. It does not say the Bible is all we need for salvation, much less that the Bible is all we need for theology; nor does it say the Bible is even necessary to believe in Christ. After all, the earliest Christians had no New Testament to which they could appeal; they learned from oral, rather than written, instruction. Until relatively recent times, the Bible was inaccessible to most people, either because they could not read or because the printing press had not been invented. All these people learned from oral instruction, passed down, generation to generation, by the Church.
As I previously responded, CA uses a straw man definition for what they imagine sola Scriptura means. It does NOT mean the Bible is all we need for salvation, all we need for theology nor necessary to believe in Christ. What Sacred Scripture IS given for us to do is so that we can have an authoritative, objective, God-breathed form of a resource so that we can know what the truth is concerning our faith. We need not rely upon human memory, human-devised traditions or man-made philosophies to know what God did for us through Christ. Like John said, these were written so that we can know we have eternal life and so that we can believe on Jesus Christ.
People DID learn of the gospel through the spoken word and they still do today, but all the truths God revealed to man - past, present and future - is found within Holy Scripture. Jesus, himself, relied upon the written word to validate His ministry, defeat the temptations of satan and reach the hearts of those who seek to know THE truth. The Bible remains today because of the power of God. Heaven and earth will pass away, but His word will never pass away.