"And this is the testimony: God has given us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life. I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life." (1 John 5:11-13)
"The one who believes in the Son has eternal life, but the one who refuses to believe in the Son will not see life; instead, the wrath of God remains on him." (John 3:36)
See: Going to Heaven - how can I guarantee my eternal destination?