For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life.(As long-time readers of this blog will know, "eternal" is not a correct translation of the Greek in this verse, but we won't focus on that in this post.)
Clearly this verse states that belief is needed for anyone to attain life.
So this restricts life to those to whom God has given belief (faith).
Now let's look at the next verse (sentence).
For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. [John 3 : 17]This verse claims that God is going to save the world, for this is the reason Jesus came.
So let's put these two claims together.
Belief (faith) is needed to attain life AND God is going to save the world (everyone).
These two claims can only be consistent with each other IF God is going to give belief to the world, to everyone.
And if he hasn't done so by the time anyone dies and leaves this planet, he must be planning to do so in the ages still ahead of us.
One of our most-quoted verses shows this link ...
For to this end we toil and strive, because we have our hope set on the living God, who is the Savior of all people, especially of those who believe.So to understand God's plan for everyone we can't afford to consider John 3 : 16 on its own. We need the next verse with it to complete the picture.
[1 Timothy 4 : 10]