Christ's ominous warning was prompted by the Pharisees' attribution of God's work, by His Spirit, to an unclean (demonic) source. In principle, we may be guilty of something similar if we are so set in our opinions that we are unwilling to acknowledge the activity of God in His other children.
The scattering of the church seems to have encouraged a change in perception from the one extreme of believing that everyone associated with the church is converted to the other extreme of suspecting that everyone who is not just like us must be unconverted. Truly, there is a fine line here, since we are required to evaluate fruit and discern what is of God and what is not. With all of the scriptural warnings about false teaching, teachers, and even brethren, we understand the necessity to compare words and deeds with the Word of God and to reject what is not of Him. We dare not underestimate the risk of deception.
On the other hand, though, another grave danger lurks in concluding that someone is unconverted because of some failing we observe in him or her. It may be that we are correct in our judgment, and our words will justify rather than condemn us. Yet, consider for a moment what is at stake if we speak idle words and misjudge this matter: It means we are attributing the work of God in that person's life—the faith, the overcoming, any good fruit—to something other than God. We may not be able to see all that He has done, but we are deciding it is nothing!
Can we grasp what transpires when we do such a thing? We are casting aspersions on the priceless Sacrifice substituted for that person. We are declaring the holy covenant that God made with that person to be null and void. We are insulting the Spirit of grace in that person's life (see Hebrews 6:4-8). Is it really worth risking that sort of evil speaking against something that is sacred?—against a beloved child of the Most High God?
Consider Paul's early experience with the church (see Acts 9). He did horrible things to holy people, and he did it with a clear conscience because he was sure he was right. He thought he was serving God by opposing the heretics—until that same God knocked him flat and told him that he was persecuting his own Maker. Decades after the fact, he was still lamenting his violence and contrariness toward people in whom the Holy Spirit dwelled. So terrible were his actions in his own sight that he did not even consider himself worthy to be called an apostle (I Corinthians 15:9). What he did was similar to what the Pharisees did in Matthew 12—he misjudged the activity of the Holy Spirit. But he also acted in ignorance, so he repented when God allowed him to see.
As Isaiah 55:8-9 says, God's thoughts are so much higher than ours. It is when we start thinking too highly of our own thoughts that we begin grieving, resisting, or even quenching the Spirit of God. God gives us these strong warnings because it is possible for us to ascend above the heights of the clouds in our own thoughts, and to arrive at the point where the mind, power, and nature of God become unrecognizable and objects of scorn. Jesus' warning should prompt us to evaluate our actions and words to ensure that we are not in any way opposing the Spirit of God.