Matthew 7:1-5 KJV
1 Judge not, that ye be not judged. 2 For with what judgment ye judge, ye shall be judged: and with what measure ye mete, it shall be measured to you again. 3 And why beholdest thou the mote that is in thy brother's eye, but considerest not the beam that is in thine own eye? 4 Or how wilt thou say to thy brother, Let me pull out the mote out of thine eye; and, behold, a beam is in thine own eye? 5 Thou hypocrite, first cast out the beam out of thine own eye; and then shalt thou see clearly to cast out the mote out of thy brother's eye.
This is a commonly quoted scripture among Christians and non-Christians alike. It is far easier to see sin in someone else's life than in our own. This is why Jesus tells us that before we speak to another person about the sin in their life, we need to examine ourselves first.
While many people will use this scripture to say that we should never say anything against another person's choices, that is not the case either. We are called to love one another and sometimes that means calling out sin in another person's life so they can come back into right fellowship with Christ. The difference is in how it is handled and whether we are in a right fellowship with Christ ourselves.