Church groups in the UK are once again joining together for a huge prayer campaign for more people to come to know God.
Justin Welby helped launch the fifth year of Thy Kingdom Come - a project which encourages Christians to pray for friends and family in the 11 days between Ascension and Pentecost.
Over one million people have taken part since its inception, and despite starting in the UK it has now spread to almost 100 countries.
Last year the Queen and the Pope gave the project their backing.
Asked at the Lambeth Palace launch what excites him about the campaign, the Archbishop of Canterbury joked: "Apart from the fact that as we pray, it changes the world...not a lot!
"The answer to all great issues that we're facing is found in the transformation of hearts. Conflict, persecution, climate change - we need human hearts to change.
"What excites me is that as we come together and pray for people to be filled with the Spirit, so they come to know and love Jesus Christ, to repent of their sins and turn away from all that is wrong and to find the love of Christ filling their hearts, we are in fact praying for the changing of our world."
This year's campaign is expected to be bigger than ever with greater efforts going into engaging families and young people.
A new resource, which has also been made for digital use, will be distributed to half a million teenagers at Church of England and Methodist schools.
Welcoming the growth of the prayer campaign, Thy Kingdom Come project director Emma Buchan, said: "It was 4 years ago today Thy Kingdom Come was launched. What 4 years it has been! What started as the seedling of an idea for a National Day of Prayer, a one off, has mushroomed into a movement where we have seen engagement in over 90 per cent of countries in the world.
"God has answered so many prayers along the way and I pray and hope that we stay utterly reliant on Him for the next 4 years!"
Of those who participated in Thy Kingdom Come last year, 70 per cent said they were more likely to share their faith with friends and family as a result.