Secret or Forbidden additions to the Bible

Different denominations have different opinions about what is and isn't Bible cannon. However, NO legitimate denomination hides their scriptures -- the only reason that these documents would be secret is because there's something fraudulent that needs to be hidden away.

Use of the phrase "True Christian"

99% of the time someone uses the phrase "true Christian" they are claiming that a 'true Christian' would agree with whatever they are saying. Their claim may actually be valid, (eg, a true Christian would not deny the existence of God) but the odds of that are low in practice. It's usually a case of "you're wrong because I said so" instead.

Use of the name "Angel of Light" to refer to God, Jesus or the Holy Spirit

This shows that Bible study isn't the group's strong point. The Bible makes it very clear that the Angel of Light is Satan, not God, Jesus or Holy Spirit. There are countless knickknacks on the market that say things along the lines of "May the Angel of Light watch over you", obviously intending to mean God or Jesus, but whomever made those seriously failed to do their research. Don't make this mistake, and avoid places that do.

No Statement of Faith

A Statement of Faith is a written description of the beliefs of a group or person. Places without one may provide one if you ask for it, but places and people that refuse to put their beliefs in writing are hiding something and are definitely not trustworthy. My statement of faith is here.

Going against scripture

If it's in the group's scripture, then something is very wrong if the group makes a contradictory claim. For example, demons clearly exist in the Protestant Bible (Jesus cast Legion out remember?). A Protestant that claims demons don't exist is therefore making a claim that is contrary to their own scripture, so something is very wrong there.

Claims that the Ten Commandments are optional

This is obviously part of the last item, but it bares repeating. There are a number of sources out there making this claim, and people who should know better buy into it. No matter who says it, the Ten Commandments are not optional, period.

Claims of divine contact that bring messages that go against scripture

No message or messenger of divine origin will ever contradict scripture. You can be 100% certain of that. Nobody is exempt from God's laws, and any spirit that says otherwise is a liar to say the least. Sadly some people claim a divine messenger gave them permission to ignore God's laws, and claim it's ok for others to do so as well.