How do we know if someone is truly of God?

In 1 John 2:19, John clarifies that those who leave the faith were never truly of us, implying that true believers will persevere in their faith. A genuine faith is characterized by a continued confession of Christ and an unwavering hope in His completed work. This perseverance is rooted in God's sovereign grace, which ensures that the elect will not ultimately fall away. Therefore, the evidence of being 'of God' is seen in one's commitment to the truth of the gospel and the continual reliance on Christ for salvation, as well as an abiding knowledge of these truths imparted by the Holy Spirit.