Concise Bible Dictionary:
This is applied in scripture to
1. The Jews’ religion, in which Paul was very strict (Acts 26:5).
2. Practical Christianity (James 1:26-27).
3. The character of the proselytes as “religious” or “worshipping” (Acts 13:43).
Strong’s Dictionary of Greek Words:
Transliteration:
threskos
Meaning:
probably from the base of 2360; ceremonious in worship (as demonstrative), i.e. pious