Question: Is it a sin to study religious documents, other than the Bible, such as the book of Mormon or even books of completely different religions. Not in the context of believing them of course, but more or less, just for the knowlege of other religions and how to disprove them. Are there any verses you could send me to help me with this decision?

Answer: While Scriptures don't speak directly to whether, or not, you should study (or learn about) other religions -- in the context of trying to disprove them -- there are a number of Biblical principles.

Some have used the following (partial) verse to say there is never any
reason to do so...

Jeremiah 10:2
Thus saith the LORD, Learn not the way of the heathen... (KJV)

In fact, the context of this verse bans learning about (or studying) the false practices of the ungodly for the purpose of practicing them. This is always wrong and is warned against in a number of passages. (See also, Lev. 18:3, 20:23; Deut. 12:30-31; passages surrounding Isa 2:6 & 2 Kings 17:8)

Other passages do warn us to be innocent of evil...

1 Corinthians 14:20
Brothers, stop thinking like children. In regard to evil be infants, but in your thinking be adults. (NIV)

Romans 16:19
Everyone has heard about your obedience, so I am full of joy over you; but I want you to be wise about what is good, and innocent about what is evil. (NIV)

Does this mean we can never know anything about it? No. We are not to be ignorant of Satan's schemes (part of being an adult in your thinking 1 Cor. 14:20). While it was directly in regards to another matter, Paul states this clearly...

2 Corinthains 2:11
... in order that Satan might not outwit us. For we are not unaware of his schemes. (NIV)

The way to do this is also shown in the Bible. Before anyone can distinguish a counterfeit from the real thing and even a lie from the truth, they must know the truth. They must know the real thing! Only in this way will you have insight from a Godly perspective.

Psalms 119:99
I have more insight than all my teachers, for I meditate on your statutes. (NIV)

Many who plunge into studying false gospels and mistruths without a firm anchor in the truth get tossed about by every 'good' sounding doctrine. The 'best' false doctrines always contain a whole lot of the truth making them hard to distinguish without a firm and expansive knowledge of God's Word. (If it's not 100% the truth, even if it's 99% the truth, it's a lie. A weapon in the hands of the one scriptures calls, 'the father of lies' - John 8:44). Before undertaking any study of false beliefs, answer these questions before God, "Do I know your Word? Do I know the doctrines of your Word? Do I hold unswervingly (Heb. 10:23) to the teachings of your Word?" If this is in doubt, your first course of study should be in God's Word.

Ephesians 4:14
As a result, we are no longer to be children, tossed here and there by waves, and carried about by every wind of doctrine, by the trickery of men, by craftiness in deceitful scheming (NAS)

Philippians 1:9-10
And this is my prayer: that your love may abound more and more in knowledge and depth of insight, (10) so that you may be able to discern what is best and may be pure and blameless until the day of Christ (NIV)

Once you are grounded in God's truth (a process that never stops, as part of sanctification - see John 17:17), you have been equipped by God to expose the darkness for what it is. Through the proper use of the truth (the light) all darkness is exposed. Armed with the truth (Eph 6:17, the 'sword of the Spirit') you will be able to expose the lies in any false belief system you study or come across.

Ephesians 5:11-16
Have nothing to do with the fruitless deeds of darkness, but rather expose them. (12) For it is shameful even to mention what the disobedient do in secret. (13) But everything exposed by the light becomes visible, (14) for it is light that makes everything visible. This is why it is said: "Wake up, O sleeper, rise from the dead, and Christ will shine on you." (15) Be very careful, then, how you live-- not as unwise but as wise, (16) making the most of every opportunity, because the days are evil. (NIV)

Yes, it is shameful to have to talk about any false beliefs (Eph. 5:12) but for the most part these acts are not done in secret - they are being publicly promoted far and wide. This makes it important - for the sake of those being led astray - that we follow what Ephesians 5:11 commands, that we 'expose' them through the light of God's Word.

Paul had learned how to make the 'most of every opportunity' (Eph. 5:16) as shown by His time in Athens (read Acts 17:21-33, especially verse 28). He had observed the not-so-hidden false practices and teaching that were common throughout the area and exposed them for the useless practice that they were - while pointing the people to the truth found in Jesus Christ. He even quoted one of their poets in the process.

In summary, know (thoroughly) and believe the truth, study and observe the false practices and beliefs around you, then expose them by using the truth.