"In the apostolic age, there was the greatest outpouring of the Spirit of God that ever was; both as to his extraordinary influences and gifts, and his ordinary operations, in convincing, converting, enlightening, and sanctifying the souls of men. But as the influences of the true Spirit abounded, so counterfeits did also abound: the devil was abundant in mimicking, both the ordinary and extraordinary influences of the Spirit of God, as is manifest by innumerable passages of the apostles' writings. This made it very necessary that the church of Christ should be furnished with some certain rules, distinguishing and clear marks, by which she might proceed safely in judging of the true from the false without danger of being imposed upon. The giving of such rules is the plain design of this chapter, where we have this matter more expressly and fully treated of than any where else in the Bible." (Jonathan Edwards)
[URL=http://www.monergism.com/thethreshold/sdg/edwards/edwards_distinguishingmarks.html]]]The Marks of a Work of the True Spirit[/URL]

This is THE battle and Satans soldiers will definitely infiltrate the ranks of believers seeking to pollute the witness of the church, by covert and insidious means. Satans weapons such as salvation by works and counterfeit faith are old and modern deceptions.

"A man may believe all the truth contained in Scripture so far as he is acquainted with it, and he may be familiar with far more than are many genuine Christians. He may have studied the Bible for a longer time, and so his faith may grasp much which they have not yet reached"
"Call the above a mere historical faith if you will, yet Scripture also teaches that people may possess a faith which is more than the product of mere nature, which is of the Holy Spirit, and yet which is a non-saving one. This faith which we now allude to has two ingredients which neither education nor self-effort can produce: spiritual light and a Divine power moving the mind to assent. Now a man may have both illumination and inclination from heaven, and yet not be regenerated. We have a solemn proof of this in Hebrews 6:4. There we read of a company of apostates, concerning whom it is said, "It is impossible . . . to renew them again unto repentance." Yet of these we are told that they were "enlightened," which means that they not only perceived it, but were inclined toward and embraced it; and both because they were "partakers of the Holy Spirit." (A.W.Pink)