John 3:16 is a verse from the Bible that states, "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life." This verse is often considered one of the most important and well-known verses in the Bible, and is often referred to as the "Gospel in a nutshell."

The book of John is one of the four Gospels in the New Testament of the Bible. 

The book of John presents Jesus as the incarnate Son of God who came to reveal the Father, to prove that He is the Messiah, and to provide eternal life to those who believe in Him. The author of the book of John emphasized on the signs and miracles of Jesus to demonstrate his divine nature and that He is the promised savior of the world.

According to William Barclay's New Testament commentary on John 3:16, the verse is one of the most famous verses in the Bible, and is often referred to as the "Gospel in a nutshell." According to Barclay, John 3:16 is a summary of the whole Gospel message, and it is a verse that sums up the whole of the Christian faith.

Barclay points out that this verse focuses on two main things: God's love and the way to eternal life. He explains that the verse states that God loves the world, and that he gave his only Son to prove that love, and that whoever believes in Jesus will not perish but have eternal life. He also states that this verse teaches us that the love of God is not something that can be earned or deserved, but is a free gift from God to all people.

He also explains that the phrase "whoever believes" is important because it means that eternal life is open to all, regardless of their background or status, and that the only requirement for receiving this gift is faith in Jesus. He concludes that this verse is the very heart of the Christian message, and that it is a message of hope and salvation for all people.

C.S Lewis, the famous Christian theologian and writer, wrote extensively about John 3:16 in his works

In his book "Mere Christianity," Lewis wrote that John 3:16 is "the most famous sentence in the New Testament and one of the most famous in the world." He explained that the verse teaches us that God loved the world so much that He sent His only Son to die for our sins, so that whoever believes in Him will not perish but have eternal life. He emphasized that the verse is the heart of Christianity, and that the message of the verse is that God's love for humanity is unconditional.

He also wrote that the verse teaches that eternal life is a gift, not something that can be earned by good deeds or works. He stated that the verse is a message of hope, and that it is a reminder that there is a way to escape eternal punishment and to have eternal life.

In his book "The Problem of Pain," he also wrote that this verse represents the core of Christianity, that God loved the world and that He gave his son Jesus to redeem the world. According to him, this verse represents the message of Christianity, that God loved the world so much that he gave his son, that whoever believes in him will not perish but have eternal life.

Ultimately, C.S Lewis felt that John 3:16 is one of the most important verses in the Bible and that it sums up the entire Christian message, that God loved the world, gave his son Jesus to redeem humanity and through faith in Jesus one can have eternal life. He also believed that this verse teaches that eternal life is a gift, not something that can be earned by good deeds or works, and that it is a message of hope.