As long as we believe, there will always be a need to defend against and a wanting to spread that belief since there is always an uncertainty when other people see differently than we do. The belief has taken on a form within us, and overtime we had mistaken it with our false identity and therefore feel defensive and aggressive when it is challenged. If we can let it die, we rest in absolute truth, there is no longer a need to take offense or defense. Truth stands alone. It will always be unaffected so therefore no need to be protected. Now this is not the truth of the Christian or of the Muslim or the Hindu etc. If it is truth within only one religion, it is not real. But It will be considered "special" Individualistic. Truth must be universal. Just as gravity affects everyone on this earth regardless of their creed. There is no drama when people talk about gravity since everyone agrees with it(if they don't, they probably had fallen off cliffs and no longer exist by now). If someone were to preach to you that gravity has a levitating effect. You would simply smile and go about your day without any disturbance in your peace of heart and mind. It must be the same with every other truth especially Absolute Truth such as God. Belief is a fear barrier, and truth is the spear that destroys that fear. Personally, i was raised Catholic, questioned myself why we get to go to heaven while everyone else is going to hell? Inquire the qualities of god and taken faith. Broke away from the rules by reading other religious books. Saw underneath the beliefs that there are truth that exist there also and at first hated Catholicism for keeping me in the dark, but ultimately had a greater respect and love for Jesus, and even more so than ever once the beliefs behind it are destroyed. Since how can you love someone truly if you believe they would banish you into hell? lol. So beliefs get in the way of actual truths And don't be mistaken, all of this wasn't easy, it was a very emotional process. So to keep things short, It all depends on the openness of the individual and their willingness to accept truth. This is where maturity comes in play and not intellectual debate.
*In a way you can say i self - preach myself XD. But if you do hear truth within someone ' s preaching it probably will be indirectly. Since it is said you can't help a person who doesn't help him/her self.