"Whoever is not with me is against me" vs "Whoever is not against you is for you"

[TonyChanYT]

Luke 9:

50b the one who is not against you is for you.”

I interpret this as the one who is not against Jesus is for Jesus.

Let proposition A = a person is against Jesus.
F = a person is for Jesus.

In terms of first-order logic:
Luke 9:50b says ¬A → F = L9b. If a person is not against Jesus, then he is for Jesus.

Formally, ¬A → F ⇒ ¬F → A. I.e., applying contraposition, we have ¬F → A = L9bC. If a person is not for Jesus, then he is against Jesus.

Now, let's turn to Luke 11:

23a Whoever is not with me is against me,

Let proposition W = a person is with Jesus.
Luke 11:23a says ¬W → A

The context of Luke 11:23 suggests the following:
If a person is with Jesus, then he is for Jesus.
I.e., W → F

Replacing W with F, Luke 11:23a says ¬F → A = L11.

Now we see that L9bC ≡ L11. Therefore, Luke 9:50b and Luke 11:23a are logically equivalent (⇔).

Whoever is not with Jesus is against Jesus
⇔ Whoever is not against Jesus is with Jesus

If you are not with Jesus, you are against Jesus
⇔ If you are not against Jesus, you are with Jesus

For another application of FOL, see The Father forgives you iff you forgive others.

If you are interested in symbolic logic, try this and this. Better yet: Take a university course in Propositional Logic (probably from the philosophy department). People trained in FOL are less likely to over-generalize and less likely to assert that Mary is the Mother of God.

See also Translating between English and Propositional Logic.