They are not of the world, just as I am not of the world. – John 17:16

Beloved brethren, if the sprinkling of the blood has really taken effect upon us, we belong, from this time forth, unto Him that died for us, and rose again. We regard ourselves as God’s men, the liveried servants of the great King-that livery the robe of righteousness. We were as sheep going astray, but we have now returned unto the great Shepherd and Bishop of souls; and henceforth we are His people and the sheep of His pasture. If any should ask, “To whom belongest thou?” we answer, “I belong to Christ.” If any enquire, “What is thine occupation?” we reply with Jonah, “I fear God.” We are not now at our own disposal, neither can we hire ourselves out to inferior objects, mercenary aims, or selfish ambitions; for we are engaged by solemn contract to the service of our God. We have lifted up our hand unto the Lord, and we cannot draw back.

In addition to this, those who belonged to God, and were dedicated to His service, were set apart and separated from others...The Lord would have those who are dedicated to Him to be separated from the rest of mankind…The Lord saith of His chosen, “This people have I formed for Myself; they shall shew forth My praise.” Before long this secret purpose is followed by the open call: “Come out from among them, and be ye separate; touch not the unclean thing, and I will be a Father unto you, and ye shall be My sons and daughters.” The church of Christ is to be a chaste virgin, wholly set apart for the Lord Christ: His own words concerning His people are these, “They are not of the world, even as I am not of the world.” Sanctify us, O Lord. Let us know, and let all the world know, that we are Thine, because we belong to Christ.” ~ C.H. Spurgeon

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