If we take John's Gospel seriously then can we ever stop unpacking the implications of 1:18?
'No one has ever seen God, but the one and only Son, who is himself God and is in the closest relationship with the Father, has made him known" (NIV 2011).
One implication of this verse is that we need to read Scripture carefully, regularly and obediently. How dare we claim to know God? Because Jesus has revealed God to us. Where do we find this revelation of God? Since the ascension of the Son to the Father, we only find this revelation in Scripture. We come to Scripture because this side of glory it is the closest we get to true, reliable knowledge of God.
Within this claim, of course, lies an understanding that the New Testament is the collection of documents which offers the fullest and most authoritative expression of Jesus' life and teaching, and its meaning for the world. To an extent we could 'get by' if, say, we had only the Gospel of John. But John's Gospel is better understood for knowledge of the other gospels and the epistles. The Old Testament's importance lies in the contribution it makes to understanding the New Testament. The latter completes the former with its hope for a better future and a fuller revelation of God; the latter cannot be completely understood without knowledge of the former.
I suggest we can go a little further re Scripture in the light of John 1:18.
If the fullest revelation of God is made known through Jesus Christ then the supreme authority over our faith - the content of what we believe - is that which makes Jesus known to us: Scripture.
Similarly, if our practice is the things we do to honour God, to make a right and fitting response to the God revealed through Jesus Christ, then such practice is subject to our knowledge of that God (and not some other god), and thus Scripture is the supreme authority over our practice.
Now, it is true that Christians at this point have divided. Some have taken Scripture's authority over practice to mean that we only do what Scripture enjoins and never what Scripture does not prohibit. Others (including Anglicans) have understood that we are free in Christ to do things which are no expressly forbidden. Yet others (including some but not all Anglicans) have felt that on the matter of practice we might also take guidance from the ancient practice of the church going back to the apostles themselves. Many an interesting debate then ensues as we debate whether (say) practice X really is forbidden by Scripture or practice Y goes back to the second century rather than to the apostles or practice Z is a fair innovation in the light of our knowledge of both Scripture and tradition.
Nevertheless it would be a brave Christian who sought to do something and claim it was God's will in the face of a majority Christian teaching that this 'something' was, in fact, forbidden by Scripture. Or, alternatively, there is a similar bravery (and foolishness) in continuing to avoid a practice which Scripture conveys a command about.
For myself, trying to be a good Anglican, while also attempting to subject my Anglicaness to the judgment of Scripture, John 1:18 challenges me in respect of practice: does my practice as a Christian, as an Anglican minister and priest assist communication of the revelation of God in Jesus Christ. Does my practice enhance our understanding of that revelation? Or does it get in the way of understanding?