Actually according to Christian doctrine it is. Matthew 28: 16-20
The Great Commission
16 Then the eleven disciples went to Galilee, to the mountain where Jesus had told them to go. 17 When they saw him, they worshiped him; but some doubted. 18 Then Jesus came to them and said, “All authority in heaven and on earth has been given to me. 19 Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20 and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age.”
"Judge Not" means it's not set upon the individual Christians to pass sentence on sinners. In other words, Jesus said for the person who was without sin to cast the first stone at the adulterous woman. So "Judge Not" means not to be the ones providing the judgement/punishment. It does not mean not to discern wright and wrong. It also doesn't mean to condone sin.
All that said, If you don't believe Christ is the savior... then all the above means nothing. However, if you know someone who does believe in Christ the savior, wouldn't you expect that he would tell you about Christ at least once rather than risk that you be damned to eternal hell without Christ, without being a believer? Otherwise that Christian is showing the he doesn't give a shit about you burning in hell. Of course he may very well be wrong about all that, but if he truly believes it.............