written by 13csande
Cathedral is a very strong trashing effect that should typically be purchased on the first turn of the game, even with a 4 or possibly even 5 copper hand. This allows you to trash a card every turn for the rest of the game, including an estate on turn 2, which greatly improves your second shuffle. Eventually, this will likely cause you to trash a good card, such as a Silver, because you have already trashed most of your starting coppers and estates. While this is regrettable, it is still well worth it because trashing your starting cards will allow you to buy more good cards faster than you otherwise would have. As such, even having to trash a few good cards you still wind up with a much better deck.
Although Cathedral is a very strong project, there are still a few instances where you should be wary of it. The three main factors to make you consider skipping Cathedral are (1) the presence of a stronger trasher, (2) a single-gain board, and (3) discard attacks.
Very few trashers are stronger than Cathedral, but a few do exist. These include Donate, Goat, and sometimes Chapel. These can allow you to get thin roughly as quickly and as efficiently as you would with cathedral, but without the downside of being forced to trash good cards later in the game. Therefore, when these other powerful trashers are available, it is often correct to skip Cathedral in favor of them.
Single-gain boards tend to be when Cathedral is at its weakest. Since you have to trash a card every turn, and can never add more than one card per turn to your deck, your deck can never increase in size after buying Cathedral on this type of board. That means that if you buy Cathedral on turn 1 your deck will never be bigger than 10 cards. This is fine at first as it allows you to quickly replace your starting coppers and estates with better cards, but eventually you will likely need to start buying provinces. At this point you will start being forced to either trash the provinces you just bought, or to trash the cards that you need to keep buying more provinces, and the deck begins to collapse. Sometimes you can trash a province from your hand, buy another province, and exhaust the province pile ending the game before your opponent can catch up, but typically you will not be able to score many points doing this (usually no more than 3 provinces). As such, it will usually not be too difficult for your opponent to reach that number of points before the game ends. It is possible that in such a situation delaying the purchase of Cathedral for a few turns can be effective, although it is still somewhat unclear how beneficial buying a delayed Cathedral is.
Cathedral is also weakened by the presence of discard attacks. Since you will have to trash a card at the beginning of your turn, you will need to keep a bad card in your hand to be trashed. Therefore, getting hit by a discard attack, like a Militia, which would normally reduce you to a 3-card starting hand, now forces you to actually start from a 2-card hand, which can be difficult. However, discard attacks by themselves are generally not enough justification to skip cathedral. The player that buys cathedral will be able to get to the point where they are consistently playing the discard attack much more quickly than a player that skips cathedral, and therefore the player that skipped cathedral will likely never be able to catch up. Discard attacks with strong alternative means of trashing can push you towards the other trasher over cathedral though. This is most evident with Mercenary, which is both a strong trasher and a discard attack, and thus usually discourages going for cathedral. On the other hand, cards that allow you to increase your hand size at the beginning of your turn (ie. Duration draw such as Wharf) are good for cathedral since they help make it more likely that a card you want to trash is in your hand at the beginning of your turn.
In conclusion, cathedral is a very strong project that you need a compelling reason not to buy on turn 1. While those reasons can exist, they are quite rare.