Both Old Testament and New Testament writers warned ravenous wolves sneaking into flocks led by false shepherds and cleverly harming the unprotected sheep. This is a fact of life throughout history. Perhaps Jude, the half-brother of Jesus said it best:
For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ. 
These “certain men” Jude was writing about were wolves who had an agenda to fleece and devour the flock. Obviously, this was happening in the church that had been planted shortly following the ascension of Jesus Christ into heaven. Very little time had passed before they had “crept in.”
But nothing is new under the sun. Paul predicted this would happen as stated in the book of Acts:
For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock. Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them. 
The apostasy in the book of Jude was certainly a fulfillment of Paul’s prediction. However, delving back into an earlier time, it is apparent that the forefathers of Paul and Jude were plagued by similar deceptive plans to dupe those who were called sheep. It would take pages and pages to exhaust the references found in the books of the major and minor prophets in the Bible with regard to those who were neglecting, abusing, and leading innocent sheep to the slaughter. As one example, we look to a plea of Jeremiah found in the 23rd chapter. He proclaimed:
Woe be unto the pastors that destroy and scatter the sheep of my pasture! saith the LORD. Therefore thus saith the LORD God of Israel against the pastors that feed my people; Ye have scattered my flock, and driven them away, and have not visited them: behold, I will visit upon you the evil of your doings, saith the LORD.  To read this entire article and for endnotes, click here.