1 Thessalonians 5:21 Prove all things; hold fast that which is good.
It's certainly a biblical principle to hold up and test all things through the filter of God's word. All Christians should understand and practice this command from the Lord.
However, this verse is used by the legalist to find something to divide over, because unity is a dirty word to them.
In the mind of a legalist, every single issue of life is a "test".
For example: In your mind, you may think you're enjoying fellowship with them. However, in their mind, they are testing you.
They are testing to see if YOU measure up to their standard. If you don't, they are quick to divide.
In the legalist's mind, they are the one's that are living God's way. So, they are constantly "testing" to see if you're as spiritual as they are. If you're not up-to-par, they will find a reason to divide.
If only the legalist had read the previous 5 verses in 1 Thessalonians 5:
1 Thessalonians 5:16-18 Rejoice evermore. Pray without ceasing. In every thing give thanks: for this is the will of God in Christ Jesus concerning you.
The balance to proving all things, is having enough Christian character to pray, rejoice and give thanks with the brethren.