The pair will meet in Las Vegas later with both boxers expected to share close to £200 million.
"The Lord will always be with me and strengthen me and deliver him into my hands," Pacquiao said as he weighed in for the fight.
The Filipino boxer wore a white t-shirt with the slogan 'All Glory and Honour Belongs to God' on the back.
The fight is set to generate an estimated $400m and be viewed by millions around the world.
The winner will take the WBC, WBA and WBO welterweight titles.
Pacquiao has previously spoke of his Christian faith.
"I have a lot of dreams and visions," he told the Guardian, "I even heard the voice of God. When I heard the voice of God, I am trembling and melting. I feel I have died. It was an amazing, amazing experience.
"I'm happy because I found the right way, salvation, born again. We are required to be born again, all of us.
"Christ said unless we are born again we cannot enter the kingdom of God. So it's very important to me.
"Jesus Christ said: 'I am the way and the truth and the life. No one comes to the Father except through me.' There is no other way. The only way is through Jesus."
The boxer had been seen as the 'bad boy' of the sport until a few years ago.
Selfie before weigh-in. #MayPac pic.twitter.com/qco7A4V7UR
— Manny Pacquiao (@MannyPacquiao) May 1, 2015"People are surprised by the way I have changed my life," he said.
"I knew I could do whatever I wanted. But I changed when I heard the voice of God, and I can testify that there is a God.
"I saw two angels, white, long, big wings. I saw Paradise. God showed me about the end of time. This all happened within the last three years. That's how I changed my life."
Pacquiao, an eight-division world champion, and Mayweather, unbeaten after 47 professional fights, have long tried to agree a deal.