Q: Is the faith of Jn. 3:15 the same as that mentioned in 14:12, and are the results available to everyone?
A: John 3:15 uses the word pisteuon, which is a participle form of the word believe, thus: “the believing one in Him has (present tense) eternal life.” This is a tremendously significant statement for at least two reasons: (1) the belief is not “of” Him or “of” some message, it is belief “in” Him – in the person of Christ, thus His identity is critical. (2) The believing one is not simply looking forward to some future possession of eternal life, but rather he has (present tense) eternal life. Let me illustrate the importance of this:
If someone believes at 9:00am, according to this text they are in present possession of eternal life. Now let’s say at 10:00am they commit some heinous sin, causing them to lose their eternal life. If this was possible, then what kind of life did they actually possess at 9:00am? It was one-hour life, since the person sinned and lost it. By definition, though, this is impossible. If I have eternal life at 9:00am, nothing I do at 10:00am can invalidate it since it is by definition eternal.
Thus, the grammar of this passage (along with a host of other passages) is a significant evidence for the eternal security of the believer. Christians, therefore, are motivated to good works not by fear of losing their relationship with their heavenly Father, but by (1) obedience and (2) a desire to walk in close fellowship.
Perhaps one more illustration: If I decide to hate my father, reject him completely, and even change my name, have I succeeded in becoming something other than his son? No. I have broken the fellowship for sure, but nothing I can do can possibly change my DNA. I am my father’s son whether I like it or not. Same goes for my relationship with my heavenly Father. I can rebel all I want, but by virtue of belief in Christ I become a child of God (Jn. 1:12), and I cannot change my spiritual DNA. Of course there are great consequences for rebellion and disobedience – including the tragic loss of fellowship, but our Father is faithful.
Now, as for John 14:12, consider that it is speaking to a very specific audience – namely only the 11 disciples. Judas had left the room before Christ began His upper room discourse, and thus he was preparing these 11 men for the special ministries for which they were called. While we can draw secondary application from this section, we have to remember the primary audience. To illustrate: Jn. 13:38 says that I will deny Christ 3 times. Or does it? No, it is clearly referencing Peter, and Jesus continues addressing the disciples specifically throughout Jn. 14-16. In short, the disciples had unique authority. Also, consider Jesus comment in 14:12 was in direct response to Philip’s request (14:8), and Jesus’ answer should be considered in that context. Finally, notice 14:22, and Judas (not Iscariot) understands the plural second person pronouns of Jesus’ discourse as referencing the disciples.
Much to consider, but short answer, the object of the faith is the same in Jn. 3:15 and Jn. 14:12, but the audience is different, and consequently so is the outcome.