Zentrum was strongly Catholic, but not conservative in the German tradition. It was not militarist and believed strongly in a restricted economic role for the state, as well as nominal democracy and minority rights, and relied on the support of the middle class. You could maybe argue that Zentrum wasn’t a liberal party in the German Empire, but in the Weimar Republic it very much was a liberal party, in the literal and abstract classical sense.

“Liberal” in this sense you might consider to be somewhat closer to the modern usage of the term “neoliberal” or “centrist” in a US framing. Liberalism, in this sense, is centered around support for capitalism and bourgeois democracy (ie democracy without decentralization of economic power). It’s the same sense that ‘liberal’ is used in various parties around the world - the Aussie Liberal Party, for example. They’re differentiated from outright conservatives and reactionaries (though many modern ‘liberal’ parties have been taken over by conservatives), as mentioned previously, by their ideological attachment to (usually regulated, though not severely) capitalism, not always shared by conservatives and reactionaries (see: bizarre modern rants by MAGA types against ‘Marxist Corporations’), and electoral democracy.

However, they tend to be extremely timid in terms of support for reforms, compromisers, and imaginationless believers in the invincibility and timelessness of extant institutions, all of which makes them extremely vulnerable to being bulldozed by fascist waves under the belief that fascists can be ‘reasoned with’ or ‘outlasted’ or that the law can protect them. Not only that, but their options in resisting fascism are necessarily restricted by their attachment to capitalism (as the wealthy then provide a non-neglible portion of their support, and the wealthy are generally not all that alarmed about fascism until about two years before fascism collapses the country entirely) and deep roots in formal government.

Even if that does describe you, there were individual liberals who resisted the Nazis. Just none amongst the actual elected officials of the liberal parties in the national legislature.