How do we know that God's grace is effective in believers?

The Apostle Paul emphasizes in Philemon 1:5-6 that the love and faith demonstrated by Philemon toward the Lord and others are evidence of the grace that has been at work in him. This aligns with the biblical teaching that genuine faith, born out of grace, will naturally result in good works as a response to God's mercy. The effectiveness of God's grace in believers' lives is thus demonstrated through their love for God and for one another, as well as their willingness to serve. The transformation of individuals like Onesimus from being an unprofitable servant to a useful brother in Christ further illustrates this principle.