Srila Prabhupada sometimes told the following story to illustrate how one may mix with nondevotees and yet keep one's devotional integrity:
Once a crocodile invited a monkey in a tree to come and ride on his back. The foolish monkey jumped down from the tree and soon found himself clinging to the crocodile's back in the middle of the river.
The monkey asked the crocodile, "Where are we going?"
The crocodile replied, "I'm going to take you home, where my wife will cut out your heart and we will eat it for lunch!"
The monkey replied, "But I left my heart back on shore in the tree. Will you please let me get it?"
The crocodile thought this was a good proposal and allowed the monkey to touch shore. But the monkey jumped into his tree and refused to accept further invitations from the crocodile.
The moral of this story: You may associate with the nondevotee, but don't give him your heart.