Why did the slave girl at the gate stop Peter but not John?

The disciple in John 18:16 is Andrew not John. People from early times until now have played fast-and-loose with tradition.

The context of all the Gospels are a context for this. First Andrew was a follower of John the Baptist, then Jesus the Messiah. He is known by the High Priest Caiaphas, for ill or good, because of the context- John the Baptist.

Andrew was always close to Peter which is why Peter speaks to him so often, i.e. at the supper in John, 13, as he lay on Jesus' breast, etc. Also, John.21:24- "… what about him." Peter's brother.

Jesus foretold that Peter was to deny Him 3 times, that is why the doorkeeper stopped Peter not the other disciple, whoever he was. And who is to say that the other disciple was not asked? Does everything that happened written?

When reading the Bible, you need to find other scriptures linking to the same context/text

First of all, the High Priest isn't John but Caiaphas: In Matthew 26, this states:

56 But all this was done, that the scriptures of the prophets might be fulfilled. Then all the disciples forsook him, and fled.

All the disciple left. The only one that followed Jesus was Peter and another disciple, known to the high priest. This disciple was not amongst the 12 as the Bible would have mentioned a name.

The slavegirl only allowed the high priest in the house and those acquainted with the high priest. The other disciple had to come back and fetch Peter. This shows that he convinced the high priests upon entering that it was ok to bring Peter in their interrogation meeting.

I hope this helps.

Peter, being the most radical follower of Jesus (he "rebuked" Jesus at Mark 8:33, he was rebuked by Jesus, and called "Satan," at Matthew 16:23, he is the one who cut off the soldier's ear at Jesus' arrest in John 18:10, and so on), was probably by consequence, also most easily recognized as a follower of Jesus, whereas John (who would seem more meek) might have been more easily recognized, in the courtyard, as an acquaintance of the High Priest.

The preceding conclusion is deduced intuitively as follows. It's merely common human nature: If you or I had been present as an ordinary member of this courtyard (as the slave-girl), distanced from Jesus, but familiar with the comings and goings of folks into and out of the courtyard, we might still notice Simon Peter, not being a common visitor of the courtyard, but perhaps as the most raucous of Jesus' followers, but would more readily know of John, who often frequented this place. Of course the disciples who were closest to Jesus would know intimately (all of them being close friends) of the remarkable fondness between Jesus and John.

Bible tells that as soon as Jesus was arrested, all His disciples fled and went into hiding. So there was this possibility that whoever known to be close to Jesus would be arrested or tried like Jesus.

Matt. 26:56 But this has happened so that the scriptures of the prophets would be fulfilled.” Then all the disciples left him and fled.

So there could be two possibilities. First: Peter was reluctant to enter inside lest he might be recognised by the people to be disciple of Jesus and land in trouble just like Jesus. Second: Peter was stopped because he was an unknown person trying to enter into a sensitive area.

In case of the “other disciple”: Like the Beloved Disciple, this other disciple is unnamed, closely associated with Peter and characterized as having special knowledge, wanted Peter to come inside to witness the trial of Jesus. Note that John says that this other disciple “knew the high priest and went into the courtyard with Jesus” indicating that in the courtyard this other disciple was not known to be a close disciple of Jesus or else he would have been in the same danger as other disciples.

Gill's commentary says:

If this Apostle is thought to be John, because he frequently speaks of himself in Gospel of John, then he is supposed to be known to the high priest, by carrying fish to his house, and selling it to him; so Nonnus says, he was known from his fishing trade and it is not probable that he was known, or could be known by the high priest, so as to have any intimacy with him; nor is it likely that he, being a Galilaean, would venture in; he was discoverable by his speech, and would have been in equal danger with Peter.

Or possibility is that this desciple was some other disciples of Christ, who had not openly professed him; one of the chief rulers that believed in him, but, for fear of the Pharisees, had not confessed him; or it may be Nicodemus, or Joseph of Arimathea, or the man at whose house Christ had eaten the passover. In the Syriac version he is called one of the other disciples; not of the twelve, but others.

Adam Clarke in his commentary on the bible states that:

For this “another disciple”, there are many conjectures who this disciple was: Jerome, Chrysostom, Theophylact, Nonnus, Lyra, Erasmus, Piscator, and others, say it was John.

Clarke however further says that:

“It is true John frequently mentions himself in the third person; but then he has always, whom Jesus loved, as in Joh 13:23; Joh 19:26; Joh 21:7, Joh 21:20, except in John 19:35, where he has plainly pointed out himself as writer of this Gospel. So in this instance verse, he has mentioned no circumstance by which that disciple may be known to be John.

Conclusion: So Peter was stopped either because he was an unknown person entering the sensitive area or Peter was reluctant to go inside courtyard out of fear that he might be recognised, yet his love for his master brought him till the gate.

Whereas the other disciple, if he is John the apostle, was either not known in the courtyard to be close disciple of Jesus as Gill,s commentary suggest or he was brave enough to face any danger unlike Peter or this other disciple was not from the twelve apostles.