I have other sheep which are not of this fold. These too I must bring in. They will listen to my voice, and there will be one flock, with one shepherd (John 10:16)
Two becoming one speaks that there is only one fold in the new covenant, and being part of this fold is not based upon the fleshly birth but of the Spirit, so both Jews and Gentiles who believe in Jesus will enter the fold where Jesus is the Good Shepherd. A sheepfold is a robust fenced enclosure to protect the sheep gathered within it. Figuratively Jesus uses the idea that we are sheep in his fold.
When comparing God's children to a flock or sheep in his fold we see he calls them by their name, and they follow him because they know his voice (John 10:3). When he brings the sheep out [of the fold] he goes before them and they follow him because they know his voice (John 10:4). They will not follow a stranger's voice because they do not trust his voice, but they know the good shepherd will protect them and care for and feed them.
Jesus often had compassion for the people because they were like sheep without a shepherd (Matthew 9:36). What we see in this story about sheep in a fold is that there is a fold, protection that exists around God's people or sheep, a sheepfold. We are told some tried to enter the fold without going through the gatekeeper which is the Father, ascending over or climbing over, which implies there are walls around the fold (John 10:1). We are told Jesus is the gatekeeper and he leads the sheep out, which implies from a protected fold, and though they might be more exposed outside the fold he goes before them.
So imagine you are a sheep within the fold, Look at the grass on the other side, it is always greener, the accuser wants us to believe. So the lamb steps outside the fold lured by its desires. Oh yeah, this is great feels so good... the shepherd will protect me I am his sheep.
Be viligant! Be alert! Your adversary the devil walks around like a roaring lion seeking someone to devour (1 Peter 5:8)
If we walk outside the fold or walk in darkness we expose ourselves to it. That doesn't mean you are not God's child only that protection is within the fold and if you deliberately go outside then watch out, the devil is there seeking sheep who do. If this were not true this warning in scripture would have absolutely no validity. If you step outside the fold, draw near, and the devil will flee.
Peter stepped outside the fold and was sifted by the devil, Jesus surely allowed this. But Peter was chosen and the lost was found. Jesus loves the sheep and cares for them. Those he appoints to care for the sheep, love him, therefore, will love and care for his sheep. But some will be wolves in sheep's clothing.
Listen to the voice of the Good Shepherd who goes before leading his sheep out into green pastures and beside still waters... He is the only good shepherd. Stay in the fold, he cares for you.