D&C 60

The Savior emphasized the importance of missionary work and the preaching of His gospel as chastened a few of the saints -

"But with some I am not well pleased, for they will not open their mouths, but they hide the talent which I have given unto them, because of the fear of man. Wo unto such, for mine anger is kindled against them.
3 And it shall come to pass, if they are not more faithful unto me, it shall be taken away, even that which they have."  (Doctrine and Covenants | Section 60:2 - 3)

Commanding that -

"...let them lift up their voice and declare my word with loud voices, without wrath or doubting, lifting up holy hands upon them. For I am able to make you holy, and your sins are forgiven you."  (Doctrine and Covenants | Section 60:7)

Again, teaching that He has the power to make men holy.  For we -

"...have been sent to preach my gospel among the congregations of the wicked; wherefore, I give unto them a commandment, thus: Thou shalt not idle away thy time, neither shalt thou bury thy talent that it may not be known."  (Doctrine and Covenants | Section 60:13)

I have to confess that at times that I feel the Lord is speaking directly to me.  In the New Testament the Savior taught His disciples -

"...whosoever shall not receive you, nor hear you, when ye depart thence, shake off the dust under your feet for a testimony against them. Verily I say unto you, It shall be more tolerable for Sodom and Gomorrha in the day of judgment, than for that city."  (New Testament | Mark 6:11)

And again in modern day, He taught -

"...shake off the dust of thy feet against those who receive thee not, not in their presence, lest thou provoke them, but in secret; and wash thy feet, as a testimony against them in the day of judgment."  (Doctrine and Covenants | Section 60:15)

We learn that the priesthood not only has the power to bless the lives of others, but it also has the power to curse others.  The power to curse is a serious matter and should be done only as directed by the Spirit.  I can't help but consider the seriousness of this matter as compared to Nephi when He was commanded to kill Laban.  It must be done as directed by the Holy Ghost.  Some don't realize that the dusting or washing of feet as a testimony is a priesthood ordinance...obviously, not a saving ordinance.

On the matter, I quote for the Doctrine and Covenants Student Manual -

“The ordinance of washing the dust from one’s feet was practiced in New Testament times and was reinstituted in this dispensation (see D&C 88:139-40; John 11:2; 12:3; 13:5-14.) The action of shaking or cleansing the dust from one’s feet is a testimony against those who refuse to accept the gospel (see D&C 24:15; 84:92; 99:4). Because of the serious nature of this act, Church leaders have directed that it be done only at the command of the Spirit. President Joseph Fielding Smith explained the significance of the action as follows: ‘The cleansing of their feet, either by washing or wiping off the dust, would be recorded in heaven as a testimony against the wicked. This act, however, was not to be performed in the presence of the offenders, “lest thou provoke them, but in secret, and wash thy feet, as a testimony against them in the day of judgment.” The missionaries of the Church who faithfully perform their duty are under the obligation of leaving their testimony with all with whom they come in contact in their work. This testimony will stand as a witness against those who reject the message, at the judgment.’ (Church History and Modern Revelation, 1:223.)” Doctrine and Covenants Student Manual, pages 130-31.)

“Cursings as well as blessings may be administered by the power and authority of the priesthood (see D&C 124:93) and include the sealing up of the unbelieving and rebellious to punishment (see D&C 1:8-9). The act of cleansing the feet as a testimony against those who reject the servants of the Lord is an ordinance of cursing and is not just a demonstration that a witness of the truth has been given and has been rejected. Through this cleansing ordinance, those who rejected the truth are on their own, and those who preached the gospel to them are no longer responsible for them before the Lord (see D&C 88:81-82). It is apparent in this and other scriptures given later in the Doctrine and Covenants that this ordinance is to be performed only when the Lord expressly commands it (see also D&C 75:20-22).” (Doctrine and Covenants Student Manual, page 50.)

I specifically note again the teaching of President Joseph Field Smith reference above -

"The missionaries of the Church who faithfully perform their duty are under the obligation of leaving their testimony with all with whom they come in contact in their work. This testimony will stand as a witness against those who reject the message, at the judgment."

Not knowing if one will eventually accept the gospel message, a missionaries testimony is sufficient as a witness.