The letter states "I am at the service of the cause of Christ", Maduro said, "and with this spirit I have asked for his help".
On Monday, Italy reportedly blocked an European Union statement recognizing Guaido as Venezuela's legitimate leader.
British Foreign Secretary Jeremy Hunt promptly followed suit, saying on Twitter he hoped "this takes us closer to ending humanitarian crisis".
"From today, we will spare no effort in helping all Venezuelans achieve freedom, prosperity and harmony", Spanish Prime Minister Pedro Sanchez said, recognizing Guaido.
"What we have seen in recent weeks, with thousands of people taking to the streets to defy the Maduro dictatorship, is a united people standing as one to say: enough", she said.
"I don't accept ultimatums from anybody", Maduro told Spanish TV channel La Sexta.
"They are trying to corner us with ultimatums to force us into an extreme situation of confrontation", Maduro said.
The act by Spain, Germany, France and Britain added to the pressure on Maduro after he defiantly rejected the EU's Sunday deadline to call a new presidential election.
In addition to European pressure, a bloc of Latin American nations plus Canada were to meet on Monday seeking to maintain pressure on Maduro.
Bud Light Super Bowl Ad Delivers Epic Surprise
Corn farmers clapped back at Bud Light following a commercial in which the beer boasted about not using corn syrup. What first appeared to be just another medieval-themed Bud Light commercial took a dark, and hilarious, turn.
He began to exercise authority for the first time this weekend, calling on the army to allow in humanitarian aid to a nation wracked by economic crisis.
French Foreign Minister Jean-Yves Le Drian appealed for an early presidential election that will ensure "the Venezuelan crisis ends peacefully".
Guaido says up to 300,000 people are "at risk of death" in Venezuela for want of humanitarian aid.
Maduro claims the U.S. pledge to deliver $20 million in aid relief precedes a military intervention.
Maduro also claimed that "the military option is on Donald Trump's table".
All eyes are now on Venezuela's military, which has so far been Maduro's main pillar of support, but there have been signs of unrest in the ranks.
Guaido is "now the only legitimate representative of the Venezuelan state".
Speaking at a pro-regime demonstration marking 20 years since his predecessor Hugo Chavez came to power, Maduro reiterated his call to bring forward elections for the opposition-held national assembly.
In an interview with Corriere Della Sera newspaper on Monday, Mr Guaido said he "will do whatever it takes" to persuade Italy to join the position of other European Union nations.