The candle, be it wax, oil (usually olive oil) or electric is an ancient custom of the Church known as a votive offering. Candles, flowers and other votive offerings are all symbolic of our both our ongoing prayers either to petition God either directly or through the intercession of our heavenly family, (our Lady and the Saints) or to give thanks for a petition answered. If you wanted to offer flowers at an altar dedicated to honor our Lady, they would cost more than a dollar, even if you grew them yourself the time and water, plant food etc would cost more than a dollar.
It is customary to also offer some sort of financial offering to the Church for the causes which she champions, feeding and clothing the poor, teaching the Faith, educating and housing those who cannot afford the basic comforts in life etc. There is also the upkeep of churches which are used to glorify God, and as a respite from the world is part of that work. While it is true that at some point the amount of money it cost to purchase the electric votives will be met, (sadly religious goods dealers charge far more for some items like candle stands and other liturgical items than one would expect) there will still be the cost of the electricity to run them or in some places new candles to replace the burned out ones. The small donation we make is both a mark of our devotion and as above supports the various activities of the Church which all need to be paid for.
Simony on the other hand is attempting to buy for a spiritual favor, we are not attempting to bribe our Lady, or God, or the clergy to get a petition answered when we light a candle. Remember the story of the Widow’s mite. If we have in abundance, we can and should offer a bit more than the dollar suggested, if we have nothing, the act of lighting a candle and petitioning or thanking God is just as valid.
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