This was preached at Pentecost 2015. John's references to the Holy Spirit means this fits well with the theme of the day.
Bob showed a picture of the Mathematical Bridge in Cambridge (http://www.bing.com/images/search?q=mathematical+bridge+cambridge&view=detailv2&&qpvt=mathematical+bridge+cambridge&id=D684DA0695F60DDF3B96DC6AD27D864EB94C3848&selectedIndex=0&ccid=2C26zYOL&simid=608038181373871797&thid=JN.GPh2LvWQ%2bE7R9G5AU9BURw&ajaxhist=0)
that initially was held together without screws or nails. In the same way, John brings various bits of the Faith together, and now brings in the Holy Spirit. (1 John 3:24). The Holy Spirit is a very important part of our faith. The Gnostics, against whom John was writing, may also have been claiming the authority of the Spirit. Discernment is important - there are false prophets who have gone into the world, set against God. The world has many guises - the hardest to detect is the religious one. There were false OT prophets who led people away from God. The role of the Holy Spirit is to bear witness to Jesus, and that he came in the flesh. Those who deny this are led by the spirit of antichrist.
John 16:13 is sometimes used to justify departure from scriptural doctrine, when it really means that the apostles would be helped to write the New Testament.
1 John 2:20 - an anointing from the Holy One - a allusion to the Spirit.
3:9 - another allusion - God's seed in us.
What does this mean in practice?
Alongside orthodoxy there needs to be orthopraxy - right behaviour. This means love for one another. What could be simpler or harder than that. We need to take heed lest we show lovelessness. Love comes from God.