I have friends, from work, from high-school, university etc. with one thing in common. They have no interest in God or religion. Is it a sin to associate with them at all.
I make sure that I don’t go to sinful places such as pubs, nightclubs, etc. but still topics come up that always test my will: sex, drugs, making gossip or fun of someone—all these are typical topics.
Should I just mostly avoid non-Orthodox people?
Concerning your question, “Should I just mostly avoid non-Orthodox people?” I would reply in the following way:
No. If you do, you may find yourself a very lonely person.
Why? We are called—commanded by Christ, in fact!—to proclaim the Good News of Salvation to everyone—especially to those who have never heard it. If Christ had only hung around with religious people, the sinful woman would have never repented, the blind would have never been healed, the hopeless would have never been given hope, Zacchaeus would have never changed his life, etc. As Christ says, “I come not to save the righteous, but the sinner.” On the day of Pentecost, the apostles would have never proclaimed the Gospel to the multitudes if they had agreed to avoid everyone who did not hold the same religious beliefs. They would have stayed in the upper room, and the resurrection of Christ would have remained their private little secret.
In short, don’t avoid those with no interest in religion or those who are not Orthodox; work to interest them in the love of God and to open them to a relationship with Him and His Holy Church.
As we read in one of the epistles, become an example to others in life, faith, humility, love.
Don’t be preachy, at least with your words. Let your actions be an example of what an Orthodox Christian—and all mankind, for that matter—is called to be, and do, and live, and experience, and hope.