Is there anyone who does not know this? I am thinking that everyone would assent to this basic truth. Most would agree to the statement not from any matter of principle or acknowledgement of fundamental truth but rather the acceptance is often rooted in a kind of mutual agreement to not condemn any particular sin or sinner. I won't call your sin out and you don't call my sin out because after all no one's perfect. The argument assumes imperfect beings have no moral standing to call out sin or sinners.
It might do well to note that not all imperfect people are the same. Some imperfect people despise their imperfections, agonize over the presence of imperfections, struggle to gain victory over the imperfections, and in general have a decided attitude against imperfections their own included. On the other hand you have a whole group of people, which happens to be the largest of the two groups who embrace imperfections, deny they are imperfections, defend and justify imperfections especially their own. In general they have a decided attitude to accept imperfections in themselves and in others.
The "no one is perfect mantra" is really a subtle way of saying the only way you can confront sin and sinners is to be perfect yourself. Clearly this is not a philosophy that is rooted in revealed truth. The bible is full of imperfect men condemning the sins of their generation. God even calls upon and appoints imperfect men with the task of condemning the sin of their generation. This was the role of prophets, it was a role often assumed by the kings and priest of Israel all whom were imperfect men. It is a baton that was passed to the New Testament Apostles and preachers, all of whom were and are imperfect men.
The message of the bible is not "no one is perfect but God loves you". The message of the bible is "repent and believe the gospel".
God calls men to repent! God uses imperfect men to call other imperfect men to repentance. God uses people from the first group mentioned above to call people from the second group mentioned above to repent.
We have devolved into a religious culture that has no moral authority to call anyone to repentance because "no one is perfect".
You can't condemn the sin of homosexuality because no one is perfect.
You can't condemn the sin of partaking of strong drink because no one is perfect.
You can't condemn the sin of lying because no one is perfect.
You can't condemn the sin of hypocrisy because no one is perfect.
You can't condemn the sin of adultery because no one is perfect.
You can't condemn the sin of fornication because no one is perfect.
The fact is you cannot condemn any sin in particular because no one is perfect.
Homosexuals are condemned not just because they have committed an act of sodomy but because they justify it and insist on normalizing it.
Drunkards are not condemned just because they have taken of strong drink but because they defend it in spite of what the bible says about it.
Liars are not censored just because they have told a lie but because they justify lying.
Adulterers are not condemned just because they have committed adultery but because they rationalize away the shame of it.
Fornicators are not condemned just because they have committed fornication but because they refuse to repent of the fornication.
The condemnation is justified when the sin is defiantly practiced.
To say "no one is perfect" as a serum to prevent personal sin from being confronted is akin to taking a larger dose of poison to overcome the remedial effect of biblical instruction.
The bible says in Hebrews 4:12-13 For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart. Neither is there any creature that is not manifest in his sight: but all things are naked and opened unto the eyes of him with whom we have to do.
It also says in 2 Timothy 3:16-17 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: That the man of God may be perfect, throughly furnished unto all good works.
And in, 2 Timothy 4:2 Preach the word; be instant in season, out of season; reprove, rebuke, exhort with all longsuffering and doctrine.
Note the instruction to believers in Hebrews 10:24-25 And let us consider one another to provoke unto love and to good works: Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.
Believers are to provoke one another to good works. Which means two things. If there are good works then there are evil works (sin). And if we are to be exhorting each other to good works then we are to be discouraging evil works.
When a person says, "no one is perfect" for the purpose of shielding themselves from scrutiny and if by that they mean you have to be perfect to address any imperfections then they in effect are saying only Jesus ever had any moral authority to confront any one about sin, or to condemn sin in general.
So when Noah confronted his generation as a preacher of righteousness he had no moral authority to do so.
When Moses confronted the sin and unfaithfulness of his generation he had no moral authority to do so.
When Nathan confronted David concerning his sin with Bathsheba he had no moral authority to do so.
When Elijah confronted Ahab and Jezebel along with their wickedness he had no moral authority to do so.
When Daniel confronted the pride and sin of Nebuchadnezzar and Belshazzar he had no moral authority to do so.
When Paul confronted sin in the churches he had no moral authority to do so.
The idea that unless morally perfect yourself you have no standing to confront sin and even at times sinners is absurd by biblical standards.
I will close on the same note with which I opened. Of course no one is perfect. But because there is universal imperfection does not mean that all the imperfect people have the same relationship to their imperfections. Some mourn over, despise, and are ashamed of their imperfection. Others announce, parade, and defend their imperfections.
To fail to make note of that difference is to be the most imperfect of all!