The word translated here with mask
(πρόφασις/prophasis) is not easy to interpret. It is used only six times in the New Testament, and the grammatical construction used in this verse is not found in any of those occurrences. The word can mean excuse
(a valid excuse; John 15:22), pretext
(a reason given insincerely or in pretence1), or pretence
(Mark 12:40; Luke 12:47). Paul is clearly not talking about a valid excuse for greed, since greed is something he condemns (Colossians 3:5). The idea of a pretence of greed
also does not make sense in this passage: why would Paul pretend to be greedy? We are left, then, with the idea of a pretext for greed.
This would mean that Paul’s outward behaviour among the Thessalonians would have appeared genuine and selfless, but in reality it was a false front,
an insincere excuse to cover up his greed. The NIV’s mask to cover up greed
expresses this idea.
5 For we never came with words of flattery, as you know, nor with a pretext for greed God is witness.