Stormont members voted by a majority of 59 to 40 against amending legislation to allow terminations in cases of fatal foetal abnormality and sexual crime.
The Christian Institute said: "It is heartening that the majority of MLAs have voted to uphold the sanctity of life today at Stormont."
Unlike other parts of the UK, the 1967 Abortion Act does not extend to Northern Ireland, where abortions are banned except where the life or mental health of the mother is in danger.
Anyone who performs an illegal termination could be jailed for life. The law has been the subject of a bitter legal battle and landmark ruling.
Northern Ireland's Human Rights Commission accused the Assembly of neglecting women's rights.
Last year Belfast High Court judge Mr Justice Mark Horner ruled that in cases of fatal foetal abnormality the mother's inability to access an abortion was a "gross interference with her personal autonomy" while a disproportionate burden was placed on victims when a sexual crime occurred.
A proposal to legalise the termination of pregnancies where the foetus cannot survive outside the womb was tabled by Alliance Party MLAs Stewart Dickson and Trevor Lunn.
Callum Webster of the Christian Institute said: "It is heartening that the majority of MLAs have voted to uphold the sanctity of life today at Stormont.
"There has been a media campaign to undermine the legal protections afforded to our unborn children, but thankfully politicians have resisted that co-ordinated pressure."
Mr Justice Horner's declaration of incompatibility did not immediately lift the ban but placed an onus on the Assembly to legislate.
A date for a legal appeal against the ruling is to be set on Friday.
Stormont's chief legal adviser, attorney general John Larkin QC, is among those opposed to changing the law and has outlined concerns that the move could breach obligations under the UN Convention on the Rights of Persons with Disabilities (UNCRPD).
Christian charity CARE in Northern Ireland spokesman Mark Baillie said: "We are delighted at the outcome of this vote and the signal it sends to wider society.
"The clear majority who rejected these amendments have sent a very clear and strong message that the lives of unborn babies should be protected.
"As has been pointed out, it is not true to argue fatal foetal abnormalities result in immediate death and parents testify to how they have cherished the time spent with their children, whether it was weeks, months or in some cases even years.
"We do need to look at what support is currently on offer for victims of sexual violence and we certainly support pastoral help for those who experience the pain of sexual crimes committed against them."