Scripture Gift Mission, an international Christian charity, has paid pound;12,500 for 300,000 cards to go to schools. They are being placed in racks in the schools by Boomerang Media which distributes free postcards through cinemas, restaurants, health clubs, universities, schools and sixth forms.
Headteachers voiced serious concerns about the move, the Church of England would not comment on it and the National Secular Society called on Education Secretary Estelle Morris to ban the postcards.
Even Garfield Smith, managing director of Boomerang Media, had reservations. "If you are putting something Christian into schools then perhaps you should be representing other religious factions too."
But Jess Bee, from Scripture Gift Mission, said: "It's amazing to get these postcards into schools after praying for it to happen. The poscard's message - that God loves people whether they're star pupils or not - is so relevant in this exam term."
Scripture Gift Mission said no pupil was under pressure to take the postcards. Boomerang Media sends between three and four million postcards, each costing about 3p, to 1,160 secondary schools in England, Wales and Scotland every two weeks.
They range from film and record promotions to ones for joining the Army and the National Reading Campaign. It is up to headteachers to decide which ones to display.
Mr Smith agreed to the religious postcard campaign after it was trailed in 30 inner-city schools. Only one school objected, saying it did not want to promote one particular religion.
David Hart, general secretary of the National Association of Head Teachers, said: "It's highly undesirable that such cards should be sent to schools."
Keith Porteous Wood, general secretary of the National Secular Society, said the postcards were shocking and deeply disturbing.