"For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life" (Jn.3:16).
Kenneth S. Wuest writes the following in regard to the Greek words translated "whosoever believes": "'whoever believes' = 'pas ho pisteuon' = relative pronoun with a participle verb functioning as a noun, lit. 'everyone who is believing'...Contrary to objectors who insist that Jn 3:16 stipulates everyone who maintains a constant state of believing as result of the phrase 'whoever believes' = 'pas ho pisteuon', the form of the verb to believe is not a present tense form but it is actually a nominative, singular, masculine, present active participle, i.e., a participle acting as a noun indicating 'one who believes' [in Christ as Savior], i.e., a believer. The participle acting as a noun does not require a perfection of continuous action such as continuous believing in order for an individual to be qualified as a believer" (Wuest, Wuest's Word Studies, Vol. 3, [Eerdmans Publishing Co, 1992], p. 67).
Everyone who is believing now (not believing and continuing to believe) already possesses eternal life and shall not perish. Besides that, anyone who believes with their heart will always believe the truth, as witnessed by John's words here:
"The elder unto the elect lady and her children, whom I love in the truth; and not I only, but also all they that have known the truth; For the truth's sake, which dwelleth in us, and shall be with us for ever" (2 Jn.1-2).