For this is the way God loved the world: He gave his one and only Son, so that everyone who believes in him will not perish but have eternal life. (NET)
Many call it the “gospel in a nutshell”. Max Lucado describes the passage in his book 3:16 as
[a] twenty-six word parade of hope: beginning with God, ending with life, and urging us to do the same. Brief enough to write on a napkin or memorize in a moment, yet solid enough to weather two thousand years of storms and questions.
Because He loves us, God has taken the initiative to rescue the people He created. Without God sending Jesus as the hero to save the day, John reminds us that we are left perishing (3:16), condemned (3:17-18), and the wrath of God (3:36) remains upon us. God has graciously provided salvation but we must receive it by believing in His Son (1:12).
What can we learn about faith from John 3:16?
The phrase “everyone who believes in him” is translated from the Greek below:
πᾶς ὁ πιστεύων εἰς αὐτὸν
The key word here is “believe” (πιστεύων), which is a participle. Participles are verbal adjectives. They can be used in a variety of ways but there are two primary categories. The first is adverbial in which the participle modifies or describes a verb. The second is adjectival. In this case it modifies or describes a noun.
In Greek the participle has a tense (or aspect) associated with it (a detailed look at verbal aspects can be found here in a paper by Andre Naselli). Outside of the indicative mood (note that the participle does not have mood) the tense does not relate to the time of the event (past, present, or future) but rather to the “kind of action” (continuous, simple, completed etc). 
Here is a table that combines info from Mounce’s Basics of Biblical Greek and Naselli’s article.
|participle aspect||Mounce description||Naselli description||note|
|present||a continuous action||the action is viewed as in progress||imperfective|
|aorist||an undefined action, or “simple occurrence”||the action is viewed as a whole||perfective|
|perfect||a completed action with present effects||the action is viewed as completed but having continuing results||perfect|
Translation guidelines from Greek grammars suggest adding -ing to the verb stem when translating a participle although context and the tense of the main verb must always be taken into account.
|participle aspect||Adverbial Translation (using Belief)||Adjectival Translation (using Belief)||# times in John (belief)|
|present||while believing||the one who is believing / the believing one||19 |
|aorist||after believing||the one who believed||2 |
|perfect||after having believed||the one who has believed||1 |
In our passage the participle (πιστεύων) is an adjectival participle because it is articular (it has a definite article (ὁ)). In this passage it is not describing another noun but actually functioning as a noun (making it a substantival participle which is a subset of the adjectival participle). Our participle is in the present tense (aspect).
This means a translation that incorporates the aspect might be “everyone who continually believes in him”, “everyone who has active faith”, or ” everyone who has enduring faith in him”.
But is John telling us that this belief must be enduring faith?
Before jumping to that conclusion we much keep in mind that the aspect of the substantival participle is considered weaker than other participle forms. That means that the continuous nature of the present tense/aspect can be diminished as Daniel Wallace reminds us with this example: 
[the present participle] in Mark 1:4 does not mean “the one who continually baptizes” but simply “the baptizer”.
The purpose of substantival participle, while functioning as a noun, is to describe the noun and provide us with its identifying characteristics. In the Mark passage it is used to tell us about John, a person known for being someone who repeatedly baptized people. In our passage, John (the author of the gospel) is describing people who have eternal life. People that are not perishing can be described as believing.
Wallace does provide a caution that although the tense/aspect can be diminished when the context requires it, that does not mean it should be ignored:
Just because a participle is adjectival or substantival, this does not mean that its verbal aspect is entirely diminished. Most substantival participles still retain something of their aspect. A general rule of thumb is that the more particular (as opposed to generic) the referent, the more of the verbal aspect is still seen.
In his grammar, Wallace provides the following notes on John 3:16 in particular:
… it seems that since the aorist participle was a live option to describe a “believer,” it is unlikely that when the present was used, it was aspectually flat. The present was the tense of choice most likely because the NT writers by and large saw continual belief as a necessary condition of salvation. Along these lines, it seems significant that the promise of salvation is almost always given to ὁ πιστεύων, almost never to ὁ πιστεύσας (apart from Mark 16:16, John 7:39 and Heb 4:3 come the closest)
However, not all agree with Wallace on this point. And if we were left with only this passage then we would have to say the grammatical evidence may lean toward enduring faith but is still inconclusive. However, other passages like Colossians 1:23 (explored in this post); 1 Corinthians 15:2; Mark 4:3-20; 1 John 4:2, 5:1; and Hebrews 10:35-39 also point to an active, ongoing, and enduring faith being a characteristic of those who have eternal life.
I agree with Daniel Wallace’s assessment. Given the different aspects that were available, John could have chosen a form of the participle that emphasized a simple decision that occurred in the past. Instead he chose the form that indicates an active, current, and present faith. Even if we deemphasize the continuous aspect, which need not mean “never ending” but rather active and ongoing from the author’s point of view, we still have people being described as active believers. In 1 John we get a picture of what an active, saving faith that allows a person to have assurance looks like to John.
An application thought
Have you ever heard the diagnostic question, have you ever placed your trust in Christ?
or have you ever believed in Jesus?
These are good and important questions. But they focus on a past decision. A point in time when someone prayed the “sinner’s prayer”. Wouldn’t a better question focus on where they are now instead of what they did in the past? Something like – Are you actively trusting in Jesus Christ today for your salvation? That question and the expectation of an active and ongoing faith, rather than a point in time decision, allows to understand where the person is today. It also seems to capture the description of the believer John paints in this most familiar passage.
 Mounce, page 135, 241, 245-247
 John 1:12; 3:15,16,18,36; 5:24; 6:35,40,47,64; 7:38; 11:25,26; 12:44, 46; 14:12; 17:20; 20:31
 John 7:39; 20:29
 John 8:31
 Mounce, page 273
 Wallace, page 620