I lean toward the Received Text as more trustworthy, but it is worthwhile considering what difference the other versions of the Greek text introduce into a passage. I am completely unimpressed with Sinaiticus and Vaticanus.
The RT goes with gar. The MGNT goes with hoti.
Gar, as I understand its present status in linguistic analysis, having researched this more, is a discourse marking particle indicating that what follows is providing further support for what immediately proceeds it: in conveys "after all" or "you see". But hoti is a subordinating conjunction that introduces a reason for what comes before or after it.
Jhn 10:24 Then came the Jews round about him, and said to him,
"How long are you keeping us in suspense? If You be the Christ, tell us plainly."
Jhn 10:25 Jesus answered them,
"I told you, and you believed not: the works that I am doing in my Father's name, they are bearing witness of me.
Jhn 10:26 But you are not believing, because ye are not of my sheep, as I said to you.
Jhn 10:27 My sheep are listening to my voice, and I know them, and they follow me. "
Received Text John 10:26-27 ἀλλ᾽ ὑμεῖς οὐ πιστεύετε οὐ γὰρ ἐστὲ ἐκ τῶν προβάτων τῶν ἐμῶν καθὼς εἶπον ὑμῖν
τὰ πρόβατα τὰ ἐμὰ τῆς φωνῆς μου ἀκούει, κἀγὼ γινώσκω αὐτά καὶ ἀκολουθοῦσίν μοι
But you are not believing Me.
After all/ You see, you are not my sheep, just as I said to you. My sheep are listening to my voice, and I know them, and they follow me. (You are not listening to My voice and following Me, so it is evident that you do not believe Me.)
i.e. You are not believing what I just told you, but called it nonsense. Why should I expect you to believe me, if I tell you something else plainly). Additional evidence that you are not believing Me is that you are not My sheep, because sheep listen to and follow their shepherd, and you are not listening to and doing what I tell you to do.
Morphological GNT 10:26-27 ἀλλὰ ὑμεῖς οὐ πιστεύετε ὅτι οὐκ ἐστὲ ἐκ τῶν προβάτων τῶν ἐμῶν
τὰ πρόβατα τὰ ἐμὰ τῆς φωνῆς μου ἀκούουσιν κἀγὼ γινώσκω αὐτά καὶ ἀκολουθοῦσίν μοι
!0:26-27 But you are not believing me,
because you are not my sheep. My sheep are listening to My voice, and I know them, and they are following Me. "
On the face of it this
could mean,
"
The reason you are not believing Me, is that you are not My sheep. My sheep listen to Me and do what I tell them to do.
i.e. "You need to become a sheep before you can believe Me. But you are not my sheep, which is why you are not believing Me. If you had become a sheep, you would be listening to My voice and following Me."
However, it could be read as an elipsis for,
" But (I can tell) you are not believing Me,
because you are not My sheep. My sheep are listening to My voice, and I know them, and they follow Me."
i.e. "
The reason (I can tell) you are not believing Me, is that you are not My sheep. My sheep are listening to Me and doing what I tell them to do. Therefore, you are not My sheep."
This would mean,
"My sheep are listening to my voice and following me: doing what I tell them to do. (I can see that) you are not believing Me, because you are not listening to Me and doing what I tell you to do. (I can see that) you are not my sheep, because you are not behaving as My sheep behave.
Both Greek texts allow for an understanding that Jesus is presenting not listening and not following/obeying as evidence for the deduction that "You are not believing Me." But The Received Text says this more directly.