I am a theist. Why is it that I become an atheist sometimes when I mix with materialistic people?

Your spiritual Samskaras are not yet strong. You are not yet moulded properly. You
need Satsang for one year more. You should repeat 200 Malas of any Mantra daily (200 x 108 =
21,600 times). You should take Sattvic food only, such as rice, Dhal, bread, fruits and milk.
Give up meat at once. Observe Mouna for two hours daily. Live alone for two hours in a room.
Introspect. Meditate. Watch your thoughts carefully.

From „… http://bit.ly/cBdWVP