Are we being easily decevied by satan?

We have a small library inside the internet cafe onboard, it is basically supported by whoever brings in books and donate it for others to read. So you can find a great deal of different stuffs in different languages, magazines, novels (mostly), and some other stuff. Couple of months ago I picked one book that caught my attention because of the title of it (Wondeful and powerful Magesty of our God). I thought, hum, at last a christian book, but it wasnt until last week that I decided to give it a look. I started reading and there was something weird, though the writer told a lot of true things and ways to behave and believe in Jesus, there was something wrong in there, until I found that he was quoting from books I never read or heard from the Bible before. It was amazing how I almost was deceived by the devil by that book. I mean, if you come to think about it, the book was full of God's truths but mixed with non-biblical material and human teachings. That's why the apostol Paul asks God to help us see when we are being deceived. It is so easy, and I almost fell for it. That brings to my mind, a friend of mine from the childhood. We were both in a christian elementary school. Her parents were into it also, not really deep, but at least they were open to it and attended for several years a christian church. After many many years, I never heard of her and her parents and one good day, I ran into them on a bus, we started talking about our lives and how we have grown up and things had changed, and then her mom told me that they were attending this church which is really a sect but they call themselves a church, and there are healing miracles and stuffs that seems like Christ-centered but if you go to their doctrines and believes you find that they are really into something else that is not Christ completely. They dont believe in the Holy Spirity and dont believe in the Trinity among other things. So she was talking to me about how she felt wonderful in that church, and how my friend felt like "at home" She related to me that no-one judged them there, and the "sermons" were always good, she was even excited to be on the choir. I was paralized to hear that, I was like "what the heck" I came to understand later that when we are not well rooted in Christ, when we dont have a knowledge of the only authoritative living Word of God, we are vulnerable to Satan's attacks to fall for this deception. The Bible is full of warnings about this very thing. "at the end of the times, a lot of people will fall from their faith because of satan's deception and phalse profets" (paraphrasing). I can only think about that whole lot of people who are being dragged away from Jesus Christ salvation and forgiveness. I can only think about my friend from my childhood, who somehow allowed satan to deceive her, I can only think about, how I am not doing so much about it, I can only think about how much God gave giving his only Son to save us and give us eternal life.