A Christian responds to atheism by acknowledging that the knowledge of God is written upon the human heart and visible in creation, yet suppressed by the sinful nature. We testify to the gospel not merely to win an argument, but to call sinners to repentance and faith in Christ, whose resurrection is the final assurance of God's reality and coming judgment.
The Scriptures teach that God has not left Himself without a witness; the heavens declare His glory and the works of creation reveal His eternal power and Godhead (Romans 1:20; Psalm 19:1-2). As the Westminster Larger Catechism Q.2 summarizes, the light of nature and the works of God plainly declare His existence. However, because we are all fallen and sinful, we are prone to suppress this truth (Institutes Ch.4 §1). Therefore, while we may present rational arguments, we must understand that the problem is not a lack of evidence, but a heart that is 'dead in trespasses and in sin' (Plain Theology for Plain People).
When engaging with atheism, we must remember our own sinful heart and the grace that brought us to faith. We are not to be combative, but to speak with gentleness and respect. As the Apostle Paul did at the Areopagus, we call all people everywhere to repent, pointing them to the resurrected Christ who serves as the ultimate proof of God's reality and the coming judgment (Acts 17:30-31). Faith is not a product of our superior logic, but a gift of the Holy Spirit who works through the hearing of the word (All of Grace Ch.12). We invite others to 'handle and see' the gospel, knowing that it is the power of God for salvation to everyone who believes.