"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.”

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 (⇔).

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.