How is it that we Christians, possessing the Holy Spirit and sometimes "filled with the Spirit" are yet susceptible to the voices of "strangers?" (Jn. 10:1-6). When Jesus said in the passage just cited that His sheep will not follow strangers, how is it that we sometimes do? The answer is two-fold.
One, it is because the voice of the false sheep, the false teachers, the false prophets is so attractive and appealing and pleasing to the human ear and ego, that even the very "elect" will hear them out for a short while (Mt. 24:24). And, two, it is because we are yet in these corruptible flesh and blood bodies.
The church at Corinth had fallen into sin in such a way through deceptive voices that some were on the brink of denying the resurrection. Chapter 15 addresses this essential matter of faith and the fact that it was being challenged by certain false teachers.
Furthermore, the Corinthians had accepted the lifestyle of one who was having intimate relations with his father's wife. They abused the Lord's Supper, the gifts of the Spirit and there were those who felt superior to those whom they saw as being inferior to themselves because they spoke in tongues and some did not. And yet the apostle could still say about these Corinthians "To the church of God which is at Corinth, to those who are sanctified in Christ Jesus, called to be saints, with all who in every place call on the name of Jesus Christ our Lord, both theirs and ours." And "...all are yours, And you are Christ's, and Christ is God's." And if you remember chapter 5 of this epistle you will recall that Paul did not deny that the one committing incest was actually a believer.
How can it be that we, the church of Jesus Christ, can not only be susceptible to such deception but fall into deception without discerning that we have? The answer? We live a life that is tainted by a corrupt nature; we are yet in a corrupt state (Roms. 8:20-22; 1 Cor. 15:50-57). And in this corrupt state we are susceptible to the deceit of the false teachers.
The only weapon against the effects of corruption for the Christian is the truth of God's word.