This section is a bold affirmation of doctrinal truth, with a resounding call to believe that truth. The absolute certainty with which John writes may sound odd or harsh to modern ears that have been trained to think such certainty unbecoming. But if God has spoken, then to be hesitant with his message is inappropriate and suggests an unwillingness to believe him. Furthermore, if his message involves the only way for people to escape eternal doom and instead find life, then clarity and boldness with this message is not callous or harsh but is the most loving thing anyone could do.
John makes clear that there is but one way to eternal life: faith in the crucified Son of God. There is no excuse for dismissing doctrine in favor of good intentions or amorphous faith. God has called us to believe certain things about his Son. Saving faith has doctrinal content. This text also strongly teaches exclusivity, the truth that salvation is found only in Christ. John’s writing makes no sense if, in the end, salvation can come by sincere belief in other religions. This truth undergirds the importance and urgency of evangelism and missions.