Jesus said:

‘I tell you most solemnly, anyone who does not enter the sheepfold through the gate, but gets in some other way is a thief and a brigand. The one who enters through the gate is the shepherd of the flock; the gatekeeper lets him in, the sheep hear his voice, one by one he calls his own sheep and leads them out. When he has brought out his flock, he goes ahead of them, and the sheep follow because they know his voice. They never follow a stranger but run away from him: they do not recognise the voice of strangers.’

Jesus told them this parable but they failed to understand what he meant by telling it to them.

So Jesus spoke to them again:

‘I tell you most solemnly,
I am the gate of the sheepfold.
All others who have come
are thieves and brigands;
but the sheep took no notice of them.
I am the gate.
Anyone who enters through me will be safe:
he will go freely in and out 
and be sure of finding pasture.
The thief comes
only to steal and kill and destroy.
I have come
so that they may have life and have it to the full.’