Krysztof Charamsa, 43, strongly criticised the Catholic Church's position on homosexuality, and said giving up his ministry was worth the help it might give other gay Christians.
Giving a press conference in Rome, Mr Charamsa said: 'I want to be an advocate for all sexual minorities and their families who have suffered in silence.'
'I dedicate my coming out to all gay priests.
'To my Church, I want to say that I reject and I denounce the current atmosphere of exasperating homophobia.
'Open your eyes to the suffering of gay people, to their desire for love.'
He also wrote a longer piece explaining his position in an Italian newspaper, where he said: "It's time the Church opened its eyes and realised that offering gay believers total abstinence from a life of love is inhuman."
The Vatican has removed Mr Charamsa from the Congregation for the Doctrine of the Faith, a group responsible for protecting traditional Catholic Church teaching. He had been a member of the group since 2003.
It also said a local bishop would decide whether Mr Charamsa would be able to carry on as a priest.
A Vatican statement said: "The decision to make such a pointed statement on the eve of the opening of the synod appears very serious and irresponsible, since it aims to subject the synod assembly to undue media pressure."
The Vatican synod on the family will discuss various issues including the amount divorced and remarried people can participate in the Catholic church and homosexuality.
The conference in Rome is due to start on Sunday 4th October and finish October 25th.
