Why would you say that? If you review each of those things, it basically boils down to 3 components:
Active Trust exercised beyond yourself (faith)...
...in an ongoing real relationship (that's not pretense)...
...that is authentic/legit content-wise (body of faith mentioned in Jude 3).
So that's real trust, a real relationship, pointed in the right direction. Even a baby can do that!
Ask any mom who is breastfeeding. That baby comes to trust Mom to be fed. The baby may not trust anybody else, but that's what the "right direction" is all about.
There's no "pretend" relationship between a baby & a mother.
The baby is there to draw nourishment & outright life from her.
The dependency is total.
The baby doesn't earn his or her right to be fed.
The baby isn't evaluated on "worthiness"; she is loved.
So, going back to your statement, do you really think such a baby wouldn't know who his or her "Mommy" is?
I tell you the truth, the man who does not enter the sheep pen by the gate, but climbs in by some other way, is a thief and a robber. The man who enters by the gate is the shepherd of his sheep. The watchman opens the gate for him, and the sheep listen to his voice. He calls his own sheep by name and leads them out. When he has brought out all his own, he goes on ahead of them, and his sheep follow him because they know his voice. But they will never follow a stranger; in fact, they will run away from him because they do not recognize a stranger's voice." (John 10:1-5)
Relationship. Sheep can relate. A dog can relate. Babies relate. All intimately know the voice of their Master or parent.
The ones who don't relate, says Jesus, are thieves and robbers who try to establish their own ladder into heaven. Self-worthiness. Perfect obedience (Matt 5:48)--I'm not saying it's not a good standard, just perfect holiness is unattainable from our direction. A temple ritual checklist. A temple recommend worthiness checklist. Doing all you can do, enough so that you "hope" grace would eventually kick in (2 Nephi 25:23). As Spencer W. Kimball says, godhood is about "pulling yourselves up by the bootstraps."
That is 100% opposite to a gift of faith--to grace--to a relationship-based salvation...and puts it all (or most) on your shoulders.
...no mortal can tell who is a Christian and who is not.
Just the opposite. No mortal can tell if he's ever done enough to earn God's approval under a sheer merit-based system. Whereas you can be certain you are in a dependent relationship with God. And you can be certain if you have stopped struggling and are trusting. A kid who is in water over his head & doesn't know how to swim or swim well knows very well if he is trusting in his parent or is trying to keep afloat all by himself.
And you can be certain that eternal life is a present-tense gift.
"Whoever believes in the Son HAS eternal life, but whoever rejects the Son will not see life, for God's wrath remains on him." (John 3:36)
"I tell you the truth, whoever hears my word and believes him who sent me HAS eternal life and will not be condemned; he has crossed over from death to life." (John 5:24)
These two verses alone penetrate the guise of Mormonism. (Because LDS define "eternal life" as godhood)
I didn't say a person couldn't tell if they themselves were Christian, under your definition, I said a person couldn't tell if someone else were a Christian. God can read the heart, man not so well. :-)