“In very truth I tell you, he who does not enter by the doorway into the fold where the sheep are, but climbs up some other way, the same is a thief and a robber.”
The sheepfold contains the waiting people of Israel, originally, to use another metaphor, ‘the lost sheep of the house of Israel' (Matthew 10:6 compare Matthew 9:36). The shepherds/teachers who do not enter by the entrance are those whom the porter will not accept, for they are not using the true entrance, and that true entrance is Jesus (John 10:7; John 10:9). The true entrance is God's way in and out which has been provided by Him in the coming of Jesus living among men. But many reject Him as the doorway and will try to win adherents by many methods rather than by submitting to God's method, and they therefore will not have the door opened to them but must enter another way and make their own doorways in. Alas for the sheep who respond to them.
And what is the sheepfold? It is the place where the sheep are gathered awaiting the call of God's messengers. It is waiting Israel, desperately looking for a deliverer but often allowing themselves to be deceived. It should be noted that the picture does not finally point to the fold but to the activities of the true Shepherd and the false shepherds. In the end the fold is of secondary importance. It is the Shepherd Who is important, along with His sheep and their daily walk with Him.
But what is the doorway? It is the God-provided way in and out through Jesus (John 10:7; John 10:9). He is both doorway and shepherd. As they live out each day they do so through the one God-approved doorway. For once Jesus came there was no other way to those who knew of Him.
But the Pharisees rejected Jesus and had to find another way in and out, a false way. It was a way of rules and regulations, a way of hardship and difficulty. It was a way that required climbing over the wall. It was a way that barred the route to many. It was a way that was even impossible for themselves. They did not use the proper entrance, God's word rightly interpreted. They tried another way.
Note the stress on the fact that there is only one true way in. Not for Jesus the idea that there are many ways, each as good as the other. He sees only one way of access and exit and that is Himself. And salvation is only found by response to the true Shepherd.