Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber. But he that entereth in by the door is the shepherd of the sheep. To him the porter openeth; and the sheep hear his voice: and he calleth his own sheep by name, and leadeth them out. And when he putteth forth his own sheep, he goeth before them, and the sheep follow him: for they know his voice. And a stranger will they not follow, but will flee from him: for they know not the voice of strangers.(John 10:1-5)
The Lord is teaching that this dignity (being a shepherd of His flock) would only belong to those who should be called by Him to the leadership of the people. Therefore He says that Himself is the Door introducing of His own will to the leadership of His rational flocks the man who is prudent and God-loving. But him who thinks himself able to take by violence and tyranny the honour that is not given to him, He calls a thief and a robber, climbing up some other way. Such were some concerning whom He speaks perhaps by one of the Prophets; They reigned as kings, and not by Me; they ruled, and not by My Spirit. And He intimates by the words before us, that if they would take pleasure in being rulers of the people they must believe and must receive through Him the Divine call to undertake this dignity, in order that they might have their rule unshaken and well established; which of course was the case with the holy Apostles, and with the Teachers of the holy Churches after them; to whom also the porter openeth. That is, either the Angel who is appointed to preside over the churches and to assist those whose lot is to minister in holy things for the good of the people, or else the Saviour Himself, Who is at the same time both the Door and the Lord of the Door. At all events, He very well asserts that the flock of sheep rightly obey and yield to the voice of the shepherd, but very quickly turn away from the voice of strangers ....
He shows Himself therefore as Lord of the fold, and Door and Porter, that they may accurately learn that they will not have their position of leadership confirmed to them, unless they come to it through Him and thus possess the God-given honour. And by adding that the sheep obey their own shepherds, but run away from strangers, He again skilfully hints that (those who do not come through the door bur climb up from another place) would never be leaders of those that should become believers in Him, but that His sheep would refuse their instruction and attach themselves to the shepherds appointed by Him.
Saint Cyril of Alexandria: Commentary on the Gospel according to Saint John, Book VI pp 64-65