"To change a person's religious system is to reconstruct the whole man himself.

When we seek to share Christ, we must be sensitive that we may seem to be attacking everything a person holds dear from childhood; all their beliefs are perfumed with domestic love, they convey to them the hopes and the fears of life, the childhood fancies, and the imaginations of manhood. Only the strongest moral natures can survive the shock of doubt that strips them of all that they have trusted from childhood. We are not just asking them to question their beliefs, but also the beliefs of all those they love; that is why humility is so important in evangelism and unless the Holy Spirit touches the heart we will never touch the mind."

Loose translation of Henry W. Beecher from The Life of Jesus the Christ.

To help understand these feelings consider how we react if someone simply brings up a doctrine that we disagree with. For example, "There is no hell" or if someone says they can show you the Trinity is not Biblical; we recoil and all manner of emotions rise up. So when we ask someone to question all they ever believed, well, it is a huge endeavor.