A person native to a culture often does not perceive the subtle influence of that culture. That all changes when a missionary enters his host culture. Everything is different, he feels, smells, senses all the differences in an overwhelming cacophony of stimuli.

These comments on the Influences of Culture are helpful:

Jesus was speaking about this danger in His comments on leaven (yeast). He warned His disciples to “beware of the leaven of the Pharisees and Sadducees” (Matthew 16:6) “and the leaven of Herod (Mark 8:15). Leaven symbolizes human imperfection (see Exodus 12:15-20, 13:3-8; Leviticus 2:11; 1 Corinthians 5:6-8). Jesus was warning against mixing imperfect human ideas with God’s truth. The Pharisees had mixed their own religious traditions with the teaching of the Scriptures; the Sadducees were the philosophers of Jewish society; and Herod represented the world system. These three influences—tradition, philosophy, and society—seem inevitably to work their way into and become part of the value system of any Christian community to such an extent that it is possible to be a Christian, but live almost entirely within a pagan value system, and not even perceive it.

This possibility began to dawn on me when we moved to Brazil and changed cultures. Culture is hardly perceived as long as we do not leave the only one we really know. A fish doesn’t perceive the water in which it swims, and neither are we aware of our culture, or the influence it exerts on our thoughts and actions. Often we must step outside of it to understand it—and to understand ourselves! - Living Proof by Jim Peterson, NavPress, 1989, p.100