Three verses I fear the most are:
Mat 12:36 But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment.
Mat 12:37 For by thy words thou shalt be justified, and by thy words thou shalt be condemned.
Speech expresses your heart attitudes and so reveals what you are as a Christian. It is how you express your Love for and relationship with Jesus and The Father in your daily life. It will either promote Christ or by not promoting Christ or His values, reject Him.
He said that if you reject Him before men He will reject you before His Father in Heaven.
So what we say is vital to our salvation as it either promotes or rejects Christ even if only by the way we do not say anything about something we know we should.
God judges us by what is in our heart and our speech confirms His judgement of our heart attitudes.
This is why we are told to use: