Why do some church candlesticks have a sharp spike?

Those sharp spikes are called prickets, and they were designed to impale large beeswax candles used in ecclesiastical ceremonies before the invention of fitted sockets. While church prickets originated for liturgical use, domestic buyers often repurpose them today for thick pillar candles displayed on sideboards and heavy mantels.

How can I tell if silver plated candlesticks are old?

Look closely at the high-wear areas around the base rims and candle nozzles to see if the underlying yellowish copper or brass base metal shows through the silver layer. Early Victorian Sheffield plated items were fused by heating silver sheet to a copper ingot, whereas later electroplating produced a more uniform but sometimes thinner metallic coating.

Can these large candlesticks be safely converted to electric lamps?

Many surviving examples have indeed been converted to electric table lamps by running internal wiring down the centre shaft, a practice seen frequently in historical interiors. When buying a converted piece, check whether the historic structure was drilled or altered during the electrical installation, as original un-drilled states generally appeal more to traditional collectors.