All the victims of the crash were members of the First Baptist Church of New Braunfels, Texas.
The collision took place at about 2.00pm on a major highway.
Texas Department of Public Safety Lt Johnny Hernandez later confirmed that 12 bus passengers died at the scene and one died later at San Antonio Hospital. A fourteenth passenger is in critical condition while the pickup driver is in a stable condition.
The National Transportation Safety Board is investigating the cause of the crash.
In a statement on Facebook, the First Baptist church asked for prayers for all involved in the incident.
The church cancelled all of their planned activities for the evening adding that the sanctuary will be open in the evening for prayer and support.
Members of the First Baptist Church comfort each other after hearing about the tragedy
Pastor Brad McLean addressed the media on Wednesday night and said members were gathering to "do what church families do."
"They're coming together, they're crying together, praying together and of course there's anxiousness as to who was on the bus and what their condition is," McLean said.
"You never know what the day is going to bring. The Lord tells us that we have today, tomorrow is not promised."