Do you know anybody who is a handwringer? Not one who uses their hands as a bell ringer, but one who expresses worry with writhing hands. John had empathy enough to see this in certain members of the ancient church and much of this letter aims at those unsettled by spiritual worry. He attempts to replace their concerns with something greater. They already have someone greater, but their thoughts, their minds, their hearts need a layering on of reminders that they might properly replace concern with truth. Look here at 1 John 4:4.
4 Little children, you are from God and have overcome them, for he who is in you is greater than he who is in the world. (1 John 4:4–ESV)
Toward their uncertainty, John sends a phrase that has taken on iconic proportions in the church. The one who is in you is greater than the one who is in the world.