Enter not into the path of the wicked, and go not in the way of evil men. Avoid it, pass not by it, turn from it, and pass away. Proverbs 4:14, 15.
(WGD 117.1)
In forming friendship, great caution should be exercised lest an intimacy be contracted with one whose example it would not be safe to imitate; for the effect of such an intimacy is to lead away from God, from devotion, and the love of the truth. It is positively dangerous for you to be intimate with friends who have not a religious experience.... Eternal considerations should come first with you. Nothing can have a more subtle and positively dangerous influence upon the mind, and serve more effectually to banish serious impressions, and the convictions of the Spirit of God, than to associate with those who are vain and careless, and whose conversation is upon the world and vanity. The more engaging these persons may be in other respects, the more dangerous is their influence as companions, because they throw around an irreligious life so many pleasing attractions.—Testimonies for the Church 3:42.
(WGD 117.2)
Like Israel, Christians too often yield to the influence of the world, and conform to its principles and customs, in order to secure the friendship of the ungodly; but in the end it will be found that these professed friends are the most dangerous of foes. The Bible plainly teaches that there can be no harmony between the people of God and the world.... Satan works through the ungodly, under cover of a pretended friendship, to allure God’s people into sin, that he may separate them from Him; and when their defense is removed, then he will lead his agents to turn against them, and seek to accomplish their destruction.—Patriarchs and Prophets, 559.
(WGD 117.3)