The claim under examination: Jesus offered eternal life as a present gift, received by believing in him, not earned by performance.
Standard · preponderance of the evidence, not proof
The verdict
That John repeatedly presents eternal life as a present gift received by believing is strongly supported, and it gives a real basis for present assurance if read in its ordinary sense. That this simple reading settles every hard passage is not yet established — which is why Christians still argue.
If you read nothing else
At his most direct, Jesus made eternal life a gift received by trusting him, not a wage earned after a lifetime of performance. That changes the assurance question entirely. The hard sayings still have to be faced, and that is the final argument.