I will focus on that last phrase. The word “insincerity or hypocrisy” suggests wearing a mask or putting on a pretense to make others think the person is speaking with authority and knowledge when in fact they are lying. In most cases, the person is deceiving themselves by believing a lie. Because what they say is not grounded in truth, they must put on a mask or pretense in order to convince others to believe what they are saying.
Such persons do not have a conscience that is governed by the truth of Scripture. That is why Paul says that their conscience is “seared” or they willfully refuse to listen to the Spirit. At the same time, they hide the system from which they are receiving their information. That system might be a person or organization, or it might be that certain Bible verses are taken out of context to support the line of teaching they espouse. These persons plant themselves in the Church and appear to be just like any other genuine believer.
The greatest protection we have against such deceptive threats to the Body of Christ is learning to listen to the Holy Spirit. Jesus reminds us that the Spirit “will guide [us] into all the truth” (John 16:13). Trust Him to guide you with the Word of God.