Security has been heightened in the wake of recent terror attacks and as a result of the former US President's presence.
80,000 people are expected to attend an open air event organised by the Obama Foundation.
Recent years have seen some unity between Catholics and Protestants and Catholic commentator Joanna Bogle tells Premier that's good news:
"Pope John Paul's visit was positive and so too Benedict's. During his visit, out of respect at a ceremony at Westminster Abbey, he uttered an amended version of the end of the Lord's Prayer."
But, she's not so sure about Obama's involvement, especially as she says he wasn't known for his rleigious views during his time in office:
"I'm less certain about the Obama celebration. It's something a bit gimmicky going on there. But, Merkel's father was a clergyman. so she probably wants to do something."
Bogle also adds that she's not keen on it being seen as something to celebrate:
"We wouldnt use the word 'celebrate' but we would say mark the date. We can pray for future unity, the Anglican bishops regretted the split and Catholics can pray with and for their brethren but Catholics are not celebating it.
"We can mark it by dialogue, welcoming people to the Vatican but not celebrate it."
She also says there's hope for the future and says she would like the next 500 years to be better than the past 500 in terms of division:
"There is lots of hope for unity."