Sanctify them through Thy truth: Thy word is truth. As Thou hast sent Me into the world, even so have I also sent them into the world…Neither pray I for these alone, but for them also which shall believe on Me through their word…that they also may be one in Us.. – John 17:17, 18; 20,21

How shall a holy God send out unholy messengers? An unsanctified minister is an unsent minister. An unholy missionary is a pest to the tribe he visits; an unholy teacher in a school is an injury rather than a blessing to the class he conducts. Only in proportion as you are sanctified unto God can you hope for the power of the Holy Spirit to rest on you, and to work with you, so as to bring others to the Savior’s feet. How much may each of us have been hampered and hindered by want of holiness! God will not use unclean instruments; nay, He will not even have His holy vessels borne by unclean hands. “To the wicked, God saith, What hast thou to do to declare My statutes?” A whole host may be defeated because of one Achan in the camp; and this is our constant fear. Holiness is an essential qualificatian to a man’s fitness for being used of the Lord God for the extension of His kingdom; hence our Lord’s prayer for His apostles and other workers: “Holy Father, sanctify them.”

Furthermore, our Lord Jesus Christ was about to pray “that they all might be one;” and for this desirable result holiness is needed. Why are we not one? Sin is the great dividing element. The perfectly holy would be perfectly united. The more saintly men are, the more they love their Lord and one another; and thus they come into closer union with each other. Our errors and our sins are roots of bitterness which spring up and trouble us, and many are defiled. Our infirmities of judgment are aggravated by our imperfections of character, and our walking at a distance from our God; and these breed coldness and lukewarmness, out of which grow disunion and division, sects and heresies. If we were all abiding in Christ to the full, we should abide in union with each other and with God, and our Lord’s great prayer for the unity of His church would be fulfilled. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/1890.cfm