"Judge not, lest ye be judged"
(aka Dann's "Sermon on the beach"1)
It's a common dilemma: You've examined a certain moral or ethical question until you're certain the answer you've come up with is correct2, and you start living according to your findings. Sometime later, you discuss your conclusion with someone else. Perhaps they agree, perhaps they don't, but how far do you go in trying to convince them of your view?
From what I can tell, the issue boils down to discerning versus judging. In a more 'religious' sense, it would be evangelism (sharing faith) versus proselytization (forcing faith). In both senses, the former seems to be the better of the two options.
In the first sense (discernment versus judgement), scripture is clear that discerning is the path to take, judging the path to avoid. When St. Paul says "Prove all things, hold fast to that which is good, abstain from every form of evil"3, he uses a word4 meaning 'to test' or 'to prove for oneself'. This sharply contrasts St. Matthew's warning: "Judge not lest ye be judged"5. Here, Matthew uses a word6, meaning 'to pick out', 'to separate', or 'to distinguish'. Both involve making a decision, but the former involves applying the conclusion to yourself, whereas the latter involves applying it to someone else.
The second sense (evangelism versus proselytization7) is where many 'evangelical' faith groups seem to take a wrong turn. Jesus says "Go ye therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit"8, using a word9 meaning 'to instruct' or 'to make disciple of'. Many groups take this to mean "stand on a street corner and force your faith on people". These folks have good intentions, just like the pharisees, but their actions aren't in line with the example Christ sets. When He was hanging out with tax collectors and prostitutes10, he didn't spend his time chastising them for their sins, he instead lovingly taught them the truth, showed them compassion, and treated them like they were members of his own family.11
In the end, both judging and proselytizing are futile and destructive. Not only can you not change someone's mind for them, God didn't intend for you to be able to. He gave all people the ability to think and reason, to one degree or another. To expect someone always to agree with you in a discussion is not only prideful, but a very selfish view of free will.12
1 -- Actually, on the last day, none of us woke up in time, so the "sermon on the beach" didn't actually happen, so I told those who wanted to hear it that I'd put it up in a node.
2 -- Whether or not you've come to the 'right' conclusion, or whether such a conclusion exists is far beyond the scope of this node.
3 -- Paul's first letter to the Thessalonians, chapter 5, verse 21
4 -- 'dokimazo' http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2327858
5 -- the Gospel of Matthew, chapter 7, verse 1
6 --'krino' http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2360004
7 -- I can never spell that word right on the first try!
8 -- the Gospel of Matthew, chapter 28, verses 19 and 20
9 -- mathe^teuo^' http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2364521
10 -- Gospel of Matthew, chapter 9, verses 12 and 13
11 -- Which, in a sense, they were.
12 -- Take these conclusions, however, not as statements of fact, but statements of what conclusions I've reached.