The German bishops issued the decree on Friday warning Catholics who stop paying the tax they would be excluded from all religious activities, also including working in a church job, becoming a godparent or taking part in parish activities.
"'Pay and pray' is a completely wrong signal at the wrong time," the reformist movement We Are Church said today. The decree "shows the great fear of the German bishops and the Vatican about further serious losses in church tax revenue".
A conservative group called the Union of Associations Loyal to the Pope asked why Catholics who stop paying the tax would be punished but those it called heretics could stay in its ranks.
"So sacraments are for sale - whoever pays the church tax can receive the sacraments," it said in a statement, saying the link the decree created "goes beyond the sale of indulgences that [Martin] Luther denounced" at the start of the Reformation.
German tax offices collect a religious tax worth 8 or 9 per cent of the annual regular tax bill of registered Catholics, Protestants and Jews and channel it to those faiths. An official declaration that one is leaving the faith frees the citizen from this tax.
Defending the decree, bishops had earlier said they were spelling out the consequences of a worshipper choosing to leave the church to avoid paying. Some Catholics had tried to remain active in their parish despite officially quitting the church.
But "it's rubbish to assume one could leave the institutional Church and remain a Catholic," said the secretary of the German Bishops' Conference.
"Whoever leaves the Church," Rev Hans Langendoerfer told the Catholic radio station in Cologne, "leaves it completely."
The annual total of Catholic church leavers, usually around 120,000, rose to 181,193 two years ago as revelations about decades of sexual abuse of children by priests shamed the hierarchy and prompted an apology from German-born Pope Benedict
Church taxes brought in about €5 billion for the Roman Catholic Church and €4.3 billion for the Protestant churches in 2010, according to official statistics.
With such full coffers, the German Church runs a large network of schools, hospitals and charity organisations at home and is one of the biggest contributors to the Vatican and to Catholic projects worldwide.
Some commentators suggested the bishops issued their decree last Friday to sidestep a looming legal case by a retired theology professor challenging the right of the Catholic Church to excommunicate those who opt out of the tax.
The German bishops had long told Catholics they would be excommunicated from the Church if they officially declared they were leaving it.
But the Vatican ruled in 2006 that a simple declaration to a tax office that one was leaving the Church was not enough to justify excommunication, Rome's stiffest punishment. The church leaver must also declare this to a priest, it said.
That prompted retired canon law professor Hartmut Zapp to file a legal case against the German Church, saying it could not excommunicate him for leaving simply to avoid paying the tax if the Vatican did not agree he deserved that punishment.
After contradictory lower court rulings, Mr Zapp's case will go on Wednesday before the Federal Administrative Court in Leipzig.
A ruling in his favour could throw into doubt Germany's whole church tax system, which was introduced in the 19th century.
However, the bishops' Friday decree, described as "excommunication lite" by the German media, could undercut Mr Zapp's case because the exclusions it listed were not described as a formal excommunication.
The German bishops are due to open their autumn plenary meeting in Fulda tomorrow, and the issue is expected to play a part in the discussions over the following three days.