For God so loved the world, that he gave his one and only Son, that whoever believes in him should not perish, but have eternal life.

 It is John's grief that so few accept the message that Jesus brought; but when a man does accept it, he attests the fact that in his belief the word of God is true. In the ancient world, if a man wished to give his full approval to a document, such as a will or an agreement or a constitution, he affixed his seal to the foot of it. The seal was the sign that he agreed with this and regarded it as binding and true. So when a man accepts the message of Jesus, he affirms and attests that he believes what God says is true. 