This is a useful site for deeper studying:
BLB - 1Jo 5: John's First Epistle - 1 John 5 (Blue Letter Bible: KJV - King James Version)
I love the text commentaries found under tools. There are a variety of options for further references to the scripture you are reading. Like this one by Charles Spurgeon regarding the scripture questioned by the original poster:
Text Commentaries: C. H. Spurgeon (Blue Letter Bible: Sermons)