Never make an intimate friend of anyone who is not a friend of God. Understand me, I do not speak of acquaintances. I do not mean that you ought to have nothing to do with anyone but true Christians. To take such a line is neither possible nor desirable in this world. Christianity requires no man to be discourteous.
But I do advise you to be very careful in your choice of friends. Do not open all your heart to a man merely because he is clever, agreeable, good-natured, and kind. These things are all very well in their way, but they are not everything. Never be satisfied with the friendship of anyone who will not be useful to your soul.
Believe me, the importance of this advice cannot be overrated. There is no telling the harm that is done by associating with godless companions and friends. The devil has few better helps in ruining a man's soul…. You may resist many open temptations, refuse many plain snares; but once you take up a bad companion, he is content. That awful chapter, which describes Amnon's wicked conduct about Tamar, almost begins with these words, "Now Amnon had a friend, a very shrewd man" (2 Samuel 13:3). JC Ryle
Taken from the book Thoughts for Young Men by JC Ryle 1886