Pope John Paul II has urged the European Union to include a recognition of Europe's Christian heritage in its first constitution, which is due to be signed next year.
The Pope said EU policy-makers should rediscover their Christian roots.
A draft constitution presented to European leaders at their recent summit in Greece referred to the "cultural, religious and humanist inheritance" of Europe, but not to its Christian tradition.
The Pope's stance is supported by Italy and Poland
The Pope has issued a lengthy document - a so-called exhortation to the Church in Europe - in which he urges those drawing up the constitution to include a reference to the religious, and in particular the Christian heritage of the continent.
He also wants churches and other religious bodies to have a recognised status as social bodies, arguing that they are far more than merely private entities.
The draft constitution was accepted by the EU leaders as a good basis for negotiation, but it now goes to a commission of member governments which could make substantial changes.
So the lobbying goes on - not just by the Vatican, but by the Greek Orthodox Church and centre-right parties in Italy, Spain, Portugal and Poland.
But there are strong arguments against endorsing any particular religion in a constitution for the Europe of the future.
Many European countries have substantial minorities who are not from Christian traditions.
And one Muslim country, Turkey, is already in the queue of countries waiting to join, with Bosnia and Albania expected to apply in the not-too-distant future.