Alternative form of people who live in glass houses shouldn't throw stones

Alternative form of let him who is without sin cast the first stone

Only those who are faultless have the right to pass judgment upon others (implying that no one is faultless and that, therefore, no one has such a right to pass judgment).

One must be careful when dealing with bad people.

Do not criticize others if you have similar weaknesses yourself.

(archaic) One should take immediate advantage of an opportunity that may not be available later.

people who dispute that a task is possible should not impede those people who attempt to do it