“See how great a love the Father has bestowed on us, that we would be called children of God; and such we are.” (1 John 3:1a) The “us” in this verse refers to born again believers in the body of Christ, the Church. Not everyone is a child of God as some believe. (John 8:44)
The things of God aren’t understood by those outside the family of God and they don’t come naturally. What does come naturally is sin and self-centeredness. Every parent knows is that you must teach a child good from bad and right from wrong. Consequently, we must learn from His Spirit how to live and act like our Father.
“And everyone who has this hope fixed on Him purifies himself, just as He is pure. Everyone who practices sin also practices lawlessness; and sin is lawlessness.” (1 John 3:3-4) If we practice sin and lawlessness we are proving through our actions that we don’t belong to the family of God. Key word here is practice. Practice is defined as what one does repeatedly, habitually.
Therefore, the proof that one is truly in the family of God is that they act like it, they love and abide with God the Father and love and have fellowship with other disciples in the body of Christ.