A Yuletide fir tree decorated with sparkling swastikas, apple- cheeked children in uniform eager to bake SS insignia-shaped cakes for the whole family, and festive Germanic candlesticks hand-crafted in a concentration camp – such were the dubious delights of Christmas under Nazi rule.

Sixty-five years after Germans celebrated the last Christmas of the Third Reich, a new exhibition at Cologne's National Socialism Documentation Centre offers, for the first time, an insight into the elaborate propaganda methods devised by the Nazis in their campaign to take the Christ out of Christmas.

The exhibition contains selected items from a vast private collection of Nazi Christmas memorabilia, including swastika and Nazi SS tree decorations, Aryan department store catalogues featuring presents for boys – toy Nazi tanks, fighter planes and machine guns – and music for carols that have been stripped of their Christian content.

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"The baby Jesus was Jewish. This was both a problem and a provocation for the Nazis," explained Judith Breuer, who organised the exhibition using the items she and her mother collected at flea markets over 30 years. "The most popular Christian festival of the year did not fit in with their racist ideology. They had to react and they did so by trying to make it less Christian."

The regime's exploitation of Christmas began almost as soon as the Nazis took power in 1933. Party ideologists wrote scores of papers claiming that the festival's Christian element was a manipulative attempt by the church to capitalise on what were really old Germanic traditions. Christmas Eve, they argued, had nothing to do with Christ but was the date of the winter solstice – the Nordic Yuletide that was "the holy night in which the sun was reborn".

The swastika, they claimed, was an ancient symbol of the sun that represented the struggle of the Great German Reich. Father Christmas had nothing to do with the bearded figure in a red robe who looked like a bishop: the Nazis reinvented him as the Germanic Norse god Odin, who, according to legend, rode about the earth on a white horse to announce the coming of the winter solstice. Propaganda posters in the exhibition show the "Christmas or Solstice man" as a hippie-like individual on a white charger sporting a thick grey beard, slouch hat and a sack full of gifts.

But the star that traditionally crowns the Christmas tree presented an almost insurmountable problem. "Either it was the six-pointed star of David, which was Jewish, or it was the five-pointed star of the Bolshevik Soviet Union," said Mrs Breuer. "And both of them were anathema to the regime." So the Nazis replaced the star with swastikas, Germanic "sun wheels" and the Nordic "sig runes" used by the regime's fanatical Waffen SS as their insignia.

Housewives were encouraged to bake biscuits in similar shapes. One of the exhibits is a page from a Nazi women's magazine with a baking recipe: "Every boy will want to bake a sig (SS) rune," proclaims the accompanying text.

The Nazification of Christmas did not end there. The Christmas tree crib was replaced by a Christmas garden containing wooden toy deer and rabbits. Mary and Jesus became the Germanic mother and child, while dozens of Christmas carols, including the famous German hymn "Silent Night", were rewritten with all references to God, Christ and religion expunged. At the height of the anti-Christian campaign, an attempt was made to replace the coming of Christ the Saviour with the coming of Adolf Hitler – the "Saviour Führer."

"We cannot accept that a German Christmas tree has anything to do with a crib in a manger in Bethlehem," wrote the Nazi propagandist Friedrich Rehm in 1937. "It is inconceivable for us that Christmas and all its deep soulful content is the product of an oriental religion," he added.

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One of the exhibition's more disturbing items is a so-called Yule lantern – a Germanic candlestick that was produced on the orders of SS leader Heinrich Himmler by the inmates of Dachau concentration camp. They were meant as gifts for faithful SS members.

Yet during the course of the Second World War, the Nazis' attempts to take the Christ out of Christmas became increasingly overshadowed by the war effort. The festival quickly turned into a scramble to send gift boxes and cards to troops at the front. By the end of the war, the Nazis had tried to turn Christmas into a ceremony of mourning for the fallen but, by then, hardly anyone took notice.

Mrs Breuer's mother, Rita, began collecting antique Christmas decorations in the 1970s. Her husband had said he wanted the sort of old-fashioned German tree that his grandmother used to have. But Rita Breuer and her daughter soon started to unearth bizarre tree decorations that had little to do with the traditional Christmas. First World War items included a miniature glass soldier carrying a hand grenade and military tree baubles in the shape of shells and tanks.

"The trend continued into the Nazi era," Mrs Breuer said. The church was too intimidated to protest, and the majority of Germans continued with the traditions they had become used to. "The Nazi Christmas ideology appears to have been adhered to most by the families of party activists who lived in towns," she added. The Nazi version of some German carols that were stripped of their Christian content survive and are still unwittingly sung by today's Germans.

The hijacking of Christmas did not end with the Nazis. There were also attempts to de-Christianise the event in the former communist East Germany. Prominent communist authors tried to substitute the birth of Jesus with that of the Soviet dictator Josef Stalin, who just happened to have been born in a humble Russian hut on December 21. "It may seem peculiar now, but in some cases the transition was almost seamless," Mrs Breuer said.

Fact or fable: What Christianity took from myth

There was an element of truth to Nazi claims that Christmas had pagan roots. When Christianity became the official religion of the Roman empire, it incorporated several ancient festivals and pagan traditions.

*Mithras The ancient festival to mark the solstice on 25 December was to celebrate Mithras, the Roman God of Light. Traditionally the festival marked the renewal of hope and is believed to have been taken up by Christians as the birthday of Jesus around the fourth century.

*Saturnalia

The customs of gift-giving, indulgent feasting and having fun comes from another Roman festival – Saturnalia – which was celebrated around 17 December. Saturn, to whom the festival was dedicated, was the Roman God of agriculture and plenty. Presents symbolised the redistribution of wealth from rich to poor during the season of greatest hardship and the rich would lay on big feasts to feed their poorer neighbours.

*New Year's Eve

Decorating the home with greenery is today seen as a Christmas tradition, but originated from the Roman celebration for New Year's Eve. Dedicated to the two-faced god Janus (who looked both forward and back), Romans would have torch-lit processions, sing songs, have their fortunes told, give presents and decorate their homes with greenery to symbolise new life.

Other ways of celebrating Christmas were adopted more recently, and were once peculiar to northern Europe.

*Yule log

Typical of these is the yule log, which is believed to have arrived in Britain only in the 17th century. One theory is that it originated in the Germanic paganism that was practiced across northern Europe before Christianity; others argue it came from Anglo-Saxon paganism, a variant of Germanic paganism practiced in England in early medieval times. The yule log was seen as a protective amulet and also a source of rivalry between neighbours.