And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God. – Romans 12:2

The more the Church is distinct from the world in her acts and in her maxims, the more true is her testimony for Christ, and the more potent is her witness against sin. We are sent into this world to testify against evils; but if we dabble in them ourselves, where is our testimony? If we ourselves be found faulty, we are false witnesses; we are not sent of God, our testimony is of none effect. I do not hesitate to say there are tens of thousands of professing Christians, whose testimony before the world is rather injurious than beneficial. The world looks at them, and says, “Well, I see: you can be a Christian, and yet remain a rogue.”… Now, what is all that? It is just this,-that the world has found out that the Church visible is not the unmixed Church of Christ, since it is not true to its principles, anal does not stand up for the uprightness and integrity which are the marks of the genuine Church of God.

Many Christians forget that they are bearing a testimony: they do not think that anybody notices them. Ay, but they do. There are no people so much watched as Christians. The world reads us up, from the first letter of our lives to the end; and if they can find a flaw-and, God forgive us, they may find very many-they are sure to magnify the flaw as much as ever they can. Let us therefore be very watchful, that we live close to Christ, that we walk in His commandments always that the world may see that the Lord hath put a difference …between you and the world. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0305.cfm