Just eight per cent of eligible couples have applied for the tax break which can be worth up to £220 a year.
The Marriage Allowance lets you transfer £1,100 of your Personal Allowance to your husband, wife or civil partner - if they earn more than you.
Under the scheme the lowest earner must make less than £11,000 a year to be eligible.
It was introduced by the government in 2015 after a campaign backed by Christian charities to encourage marriage.
Speaking at the time former Prime Minister David Cameron said: "This policy is about more than pounds and pence. It's about valuing commitment.
"Families are the bedrock of our society... this tax change is about saying as a society, we recognise that."
But with just eight per cent uptake, Christian charity CARE is calling on the application process to be simplified.
"At the end of the day if people are struggling to work themselves through the process then you can't blame them if they're not going to take advantage of this," said CARE spokesman James Mildred.
Speaking to Premier he called on the government to make it easier for people to apply.
"You have to make sure that if you are providing this sort of benefit that there are clear examples and explanations so that married couples can take full advantage.
"That's why it was reintroduced, it was reintroduced to help married couples."
He added that although £220 a year was "not insubstantial" it was "nowhere near enough".
"There's not a big enough financial incentive here so our argument would be that it needs to be massively increased," he said.