I was nodding off during the film on Wednesday night so can someone just fill-in a few gaps for me ?

Was she possessed by a demon or someting ?
What was she writing on the wall ?
What happened at the end ?

Ooo, that was a bad film. So bad, I can't remember the answers to any of your questions. Also, nor can the IMDB :D

She was possessed by Jesus himself. She'd become possessed by buying the rosary of the priest at the start, as non-religious people don't normally get the stigmata.

The basic idea was that JC was trying to spread a new gospel, saying people didn't need a church to speak to God. This would obviously have destroyed the church so they did all they could to stop it.

Frankie is a messenger of Father Alameida's spirit (and he also suffered stigmata wounds). The spirit releases Frankie (Patricia Arquette) and then Andrew Kiernan (Gabriel Byrne) goes to the church in Brazil and discovers the Gospel of Thomas (that the three priests were translating)hidden beneath the church floor. In the DVD alternate ending, Frankie dies.

http://www.ruinedendings.com/film1234ending