It has taken 20 years for St Teilo’s Church to be rebuilt at the National History Museum in St Fagans, after being moved from its original site in Pontarddulais near Swansea.
The church has been restored to its appearance in 1520
Dr Williams, himself from Swansea, has said the church's restoration was an "amazing achievement" and a "real triumph for the country".
He said, "Our history didn't just begin when they discovered coal, our history didn't end when the Romans left.
“There's a long period in-between, when we were part of Europe and a big flourishing culture," said the archbishop.
Traditional methods were used to build what according to the museum is the UK's first authentic reconstruction of a medieval masonry-built church. It contains all the elements of a late medieval Catholic church, including a rood screen and a loft, elaborately carved from oak.
Rev John Walters, the vicar of the church, led the service by inviting those who knew the church in its original location.
Rev Walters said: "The church had not been used regularly since 1850, apart from three services a year during the summer months.
"Eventually with slates being stolen from the roof it was decided that something had to be done," he explained.
