Leaving that for a moment, if Paul had written, "Test yourselves to see if you are in the faith" (2 Cor 13:5) on the Internet, I wouldn't be surprised if he had included a link to John's first epistle, because that missive seems to be a large "test of faith", filled with "if" and "if not" and picking up in sharp and glaring terms those who are and who are not in the faith. However, with this weight of "are or are not in the faith", it seems like 1st John is ambivalent. (See? I got back to that.) He seems to write with fervor over testing yourself and with confidence that those who are in the faith will be. Here, take a look.
In the first chapter John asks if we walk in darkness or in the light as tests of whether we have fellowship with Christ (1 John 1:6-7) and whether or not we confess our sin (1 John 1:8-10). Notice, however, that John bounces back and forth on that latter point -- "If we say that we have no sin" followed by "If we confess our sins" followed by "If we say that we have not sinned." And doesn't end there. He goes on to say, "If anyone sins ..." (1 John 2:1). So, which is it? Is sin continuously in need of confession, repentance, and forgiveness, or do we have an Advocate? Ambivalent.
There are lots of other "tests".
By this we know that we have come to know Him, if we keep His commandments. (1 John 2:3)And that's just the second chapter. It goes on and on. Lots and lots of warnings, contrasts of genuine verses false believers, that sort of thing. It's full of it.
The one who says he is in the Light and yet hates his brother is in the darkness until now. The one who loves his brother abides in the Light and there is no cause for stumbling in him. (1 John 2:9-10)
Do not love the world nor the things in the world. If anyone loves the world, the love of the Father is not in him. (1 John 2:15)
Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also. (1 John 2:23)
The epistle is littered with these "ifs", like a test to see if you're a genuine Christian or not. Yet John, speaking of false teachers who "go out from us" (not outside), writes, "They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us." (1 John 2:19) There isn't much ambiguity there. John appears to hold that those who are "really of us" will remain -- no question. In a letter dotted with test questions for your faith, John appears to be quite confident that those who are really in the faith must remain in the faith. Ambivalent.
In the third chapter we see something that, on one hand, appears to be quite daunting and, on the other, quite assuring. I've commented on this before.
No one who is born of God practices sin, because His seed abides in him; and he cannot sin, because he is born of God. (1 John 3:9)On one hand you have to ask yourself, "Do I practice sin?" and that might be a scary examination. On the other hand, to the one who is born of God, one of the most troubling questions of all is clearly answered. "Can I lose my salvation?" No! If practicing sin is the means of losing salvation, those born of God cannot do it. It isn't that it's not likely. It is impossible. Those born of God lack the capacity.
First John is full of these two poles. "Are you in Christ?" and "The outcome of those who are once in Christ is certain and settled." It looks ambivalent -- pulled in two directions. It isn't. It actually settles things. You might be wise to go through this and ask yourself the questions. It's good to know, one way or the other.