Sarah Mbuyi, 31, made the comment at a nursery in West London when asked by the woman if she would be welcomed at church and if God approved of civil partnerships.
The tribunal was told how on 6 January 2014 her colleague started a conversation about Christianity and asked whether she would be welcomed at church and whether God would approve of her civil partnership and allow her to marry in church.
The nursery nurse replied: "God is not okay with what you do" but that "everyone is a sinner and God offers forgiveness".
Miss Mbuyi recalled: "When I said 'No, God does not condone the practice of homosexuality, but does love you and says you should come to Him as you are', she became emotional and went off to report me to my manager."
After a complaint the woman was investigated and sacked for gross misconduct within three days for breaching the nursery's equality policies and harassing her colleague.
The woman took her case to an appeal, supported by the Christian Legal Centre, but lost and then commenced a claim at the Watford Employment Tribunal.
The tribunal agreed she had been discriminated against because of her Christian belief that the practice of homosexuality is a sin.
She had argued she had the right under EU law to enter into conversations with adult colleagues subject to the normal principles of engagement in speech.
A panel chaired by Judge Broughton, found unanimously in Miss Mbuyi's favour.
It said that it recognised the employer was "not anti-Christian" but that the woman had not been treated fairly and that the decision to sack her may have been made on "stereotypical assumptions about her and her beliefs".
Miss Mbuyi's belief was described by the Tribunal as one which is "worthy of respect in a democratic society, is not incompatible with human dignity and is not in conflict with the fundamental rights of others".
Judges added that the employer's policy that there was a "prohibition on employees expressing adverse views on homosexuality and/or describing homosexuality as a sin" would have a "disparate impact on Christians holding similar views to Miss Mbuyi on the biblical teachings on practising homosexuality."
It added: "That is not merely because a significantly higher proportion of Christians would hold such views but also because many evangelical Christians feel their faith compels them to share it."
The tribunal found there was little or no evidence to suggest that Miss Mbuyi targeted her colleague in an attempt to force her faith on her and the issue of gay people was not raised by her.
The Tribunal said the internal investigation by the employer was hampered by the "stereotypical assumption about evangelical Christians" and that the employer either "pre-judged the outcome accepting unchallenged evidence that supported the stereotypical assumption and/or interpreted Miss Mbuyi's evidence in an almost impossible way".
Andrea Minichiello Williams, barrister and CEO of the Christian Legal Centre said: "This is a brave judgment and comes as a great relief to Miss Mbuyi and to all of us at the Christian Legal Centre.
"This judgment is a 'common sense' judgment which shows understanding of the Christian faith and Miss Mbuyi's freedom to live and speak it out in the work place.
"We have been in the employment courts for over a decade now and at last we have a sensible decision."
Miss Mbuyi is now working successfully as a nanny elsewhere and said: "I only ever responded to questions that my colleague asked me and wanted the very best for her. I give glory to God for the decision and say 'well done' to the Christian Legal Centre.
"I hope that my previous employer and colleagues are well and will understand from this that my intention was for their best."