John 3:16 is usually interpreted as a purpose statement for the four Gospels, emphasizing God's love as the primary motivation for sending his son Jesus Christ as a sacrifice. John 3:16 is often used by Christians to explain the requirement for obtaining salvation.
John 3:16 was spoken by Jesus Christ to the Jewish rabbi Nicodemus. The statement was made after mentioning Moses and how he constructed a copper serpent so that those who looked upon it could be cured of a plague. Therefore, Jesus used this story to emphasize that his sacrifice would also be life-saving, but only to those who believed in his sacrifice. Considering the concept of faith expressed in the rest of the Gospel of John, it can be inferred that belief in Jesus' sacrifice must be constant.