John 3:16 is well–loved for good reason—it’s the gospel in a nutshell. If someone believes it, he has everlasting life. 

Of course, you don’t have to understand and believe it all to be born again. It’s possible to believe that promise in a basic way without having a very deep understanding of what it means. But lest we take John 3:16 too lightly, it’s probably good to remember that every word in that verse is thick with meaning:  

For God (that is, the one God of Israel, the Creator of heaven and earth, the God of Abraham, Isaac, and Jacob, who promised to send the Messiah; indeed, the Holy Trinity) 

so loved the world (indicating God’s perfectly good character of love, and demonstrating His redemptive plans for an implicitly fallen world) 

that He gave (through the Incarnation of the Son of God, who came with the purpose of dying on the cross as the Lamb of God) 

His only begotten Son (implying, all at once, the virgin birth, the resurrection from the dead, the Davidic kingship, and the eternal Trinitarian Sonship of Jesus Christ) 

that whosoever (implying the free will of man, and the universality of both the extent of the atonement and of the offer of salvation) 

believeth (understood as a single act of persuasion, and implying the ability of the unregenerate to believe) 

in Him (that is, in Christ, especially His offer of salvation) 

shall not perish (guaranteeing the eternal security of believers, while threatening eternal torment in the Lake of Fire for unbelievers) 

but have everlasting life (implying the regeneration of the believer, his future bodily resurrection, and immortality in the Messianic Kingdom and in the age to come). 

I think a child could understand the basics of John 3:16, while a theologian would be hard–pressed to plumb the depths of it. And both can believe that promise and have everlasting life. 

In the past, we’ve often had to guard against those who require too much understanding to be born again. Lately, we’ve had to defend against those who prohibit too much understanding to be born again, as if believing too much truth about Jesus, or thinking you must believe too much truth about Jesus, means you are not believing in Jesus (I know, I know, that is a very complicated sentence!).

The saving power is not in the relative “thickness” or “thinness” of your understanding, but in Jesus’ faithfulness and authority to keep His promise to the one who believes in Him for eternal salvation. If you are believing in Jesus for everlasting life, you have it. Period.