Jesus tells us that, "Those things which proceed out of the mouth come from the heart, and they defile a man. For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies." (Matthew 15:18-19) But where do they orginate? The Apostle Paul tells us that believers deal with arguments and thought (II Corinthians 10:5) and anxiety (Philippians4:6). The Apostle Peter tells us that believers deal with various cares (I Peter 5:7) and Jesus tells us that believers deal with worry (Matthew 6:31), the opinions of man and the lies of the devil (Matthew 16:13, 23) to name a few things that affect the believers mind.
All of these things work in the believers mind and can have an adverse affect on their very core, their heart. Medical science is also discovering that negative thinking can cause physical ailiment and emotional distress that begin in the mind but finds its way to the seat of a persons very being, the heart. So now we can understand better what Jesus meant in Matthew 15:18-19. So when the heart is infiltrated from negative thought of the mind, out of our mouth proceeds negative speech or confession.
James 3:6 tells us, "And the tongue is a fire, a world of iniquity. The tongue is so set among our members that IT DEFILES THE WHOLE BODY, and sets on fire the course of nature; and it is set on fire by hell." What an impact the tongue has on the believers entire being. However, Hebrews 10:23 says, "Let us hold fast the profession of our faith without wavering; (for he is faithful that promised;)" Believers are to have a good confession, a positive confession, a confession filled with faith. But if a believers mind hold fast to thoughts that breed doubt then their hearts (what they believe) will also be tainted and the natural course will be a confession of doubt and not faith.
There is something that can be done. Paul tells believers to "refute arguments and theories and reasonings and every proud and lofty thing that sets itself up against the [true] knowledge of God; and lead every thought and purpose away captive into the obedience of Christ" (II Corinthians 10:5) As believers "cast down" these things of the mind and replace them with thing above, they can expect to there confession to be one of faith and not doubt. When this happens believers will be imparting grace to those who hear their speech, that is, their confession.