For God so loved the world, that He gave His only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
This verse, written by the Apostle John, sums up Christianity. Just as Abraham was willing to sacrifice his son, Isaac, to God (Genesis 22), God gave up his son as a propitiation, or sacrifice of atonement, so that any man (or woman) could seek forgiveness for sin(s), merely by asking.
Who do we believe in to get everlasting life? Jesus, God's only begotten son--meaning a son who was not created, but actually came from God.
What is everlasting life? It is the continuance of our existence, for all eternity. This implies that those that do not believe in Christ will not continue on forever. Rather, when they are cast into the lake of eternal fire (Revelation Chapter 20) they suffer a second, permanent death.