Passage
That whosoever believeth in him may not perish, but may have life everlasting.
John 3:15 (DRB)
Nearby Context
John 3:13 And no man hath ascended into heaven, but he that descended from heaven, the Son of man who is in heaven.
John 3:14 And as Moses lifted up the serpent in the desert, so must the Son of man be lifted up:
John 3:15 That whosoever believeth in him may not perish, but may have life everlasting.
John 3:16 For God so loved the world, as to give his only begotten Son: that whosoever believeth in him may not perish, but may have life everlasting.
John 3:17 For God sent not his Son into the world, to judge the world: but that the world may be saved by him.
Study Lenses
The verse centers on "believeth", "whosoever", "perish", "life", and "everlasting". It is saying that the verse draws attention to "believeth" and "whosoever", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 14's "And as Moses lifted up the serpent..." into verse 16's "For God so loved the world as...", so "believeth" and "whosoever" belong inside that flow. In Jesus Explains God's Saving Love, the local focus is new birth, eternal life, belief and unbelief, and God's saving love.
A plain takeaway is to answer the verse's own emphasis on "believeth" and "whosoever" with trust shaped by these words, not by a vague optimism outside the passage.