Believe. That's the one condition.
John uses the word believe 98 times — more than the other three Gospels combined (34 times), and more than six times the rest of the New Testament (16 times). It's the word he's built his entire Gospel around. Believe means trust in someone, and it assumes you're convinced that what he said is true — you can't believe in someone without believing that what he claims is true; the two aren't really separable. Faith, in John's Gospel, is trusted reliance on Christ's promise to give eternal life to those who believe.
What does believing actually get you? Not a probationary status, not a maybe — eternal life, present tense, right now. He who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life (John 5:24). That's not a future hope you're waiting to find out about. It's a settled fact the moment you believe.
John pictures that one act of believing six different ways, so no one mistakes it for something complicated. Look — as Moses lifted up the serpent, so the Son of Man was lifted up, that whoever believes in Him would have eternal life (John 3:14-15). Hear — My sheep hear My voice, and they follow Me (John 10:16, 27). Enter — I am the door; if anyone enters through Me, he will be saved (John 10:9). Drink — whoever drinks of the water I give will never thirst (John 4:14). Come — the one who comes to Me I will certainly not cast out (John 5:40; 6:37; 7:37). Receive — as many as received Him, to them He gave the right to become children of God (John 1:12). Six pictures, one act: simple trust in what Christ has promised.
Simple trust in Christ's promise. That's the whole condition — nothing hidden, nothing added.
Scripture quotations taken from the New American Standard Bible® (NASB), Copyright © The Lockman Foundation. Used by permission, www.lockman.org.