Romans 14:23
Romans 14:2323But the one who has doubts is condemned if he eats, because his eating is not from faith; and everything that is not from faith is sin.
Watchtower commentary takes ("everything that is not based on faith is sin") as motivating Witnesses to abstain from anything their conscience cannot affirm in faith — including holidays whose pagan origins might trouble a sincere believer's conscience. The verse is treated as supporting the JW practice of abstaining from Christmas, Easter, birthdays, and other observances on conscience-grounds.
V23 is the closing principle of Paul's whole conscience argument — and the chapter to which it belongs (Romans 14) permits both observance and non-observance "to the Lord." The verse means: do not act against your own faith conviction. It does NOT mean: refuse all observance because someone else's conscience is troubled by it. The Watchtower's reading inverts the principle. If a Christian celebrates Christmas in good conscience, in thanksgiving for the Incarnation, that observance is "based on faith" and is not sin. If a Witness refuses Christmas in good conscience, that abstention is also "based on faith." Romans 14 consistently respects both consciences — it is the Watchtower's organisational mandate to refuse, not the individual's conscience choice, that conflicts with Paul's rule.