In 2011, weekend Mass attendance was 245, the archdiocese said, down from 344 in 2007. St. John had 635 regular attendees at Mass in 2011, up from 600 in 2007.
Parishioners learned of the merger through letters sent to them and in announcements at weekend Masses.
The decision to merge was based on numerous factors, the archdiocese said, including demographic shifts and having two parishes serving "a limited geographic area."
No one at either of the parish offices could be reached for comment Sunday afternoon.
But one longtime parishioner, Everette Slusher, who lives near All Saints on Buckius Street, said the marriage of the two churches wouldn't be easy - even though he said what's really important is going to church to pray, "not to play favorites."
"There's been a type of rivalry between the two churches for a good while," Slusher said. "I think there's a lot of people that are registered at All Saints that really don't want to be involved with St. John."
The archdiocese has been undertaking what it calls a parish pastoral planning area initiative since 2011 to deal with a dwindling Catholic population, lower attendance at Masses, and financial challenges.
In January and February, as part of the initiative, the archdiocese announced parish mergers in North Philadelphia and West Philadelphia.
The first round of parish mergers flowing from the initiative was last year, the archdiocese said, in Coatesville and Phoenixville and in the Germantown, Harrowgate, and Manayunk sections of Philadelphia.
The impending parish merger was discussed in a May 15 posting on an All Saints Bridesburg blog.
"Certainly you are aware of the priest-assignment implications of the mergers: we priests surely cannot all stay put in parishes that are no longer fully functioning," the blog said. "It becomes a human resource management nightmare of dramatic proportions."
The archdiocesan announcement said pastors and lay leaders from the merging parishes would establish a committee to help guide the transition.
The pastor of the new parish will be named at the end of the month.
Though the All Saints church building will remain open for now, all of the parish's assets and debts will be assumed by the merged parish.
Another All Saints parishioner, Steve Indan, 55, was pragmatic about the switch.
"I have no children, my daughter is grown and in college, and any merger that saves this city or saves the church money is good for me," he said. "Really, what are you going to do?"
Contact Carolyn Davis at 610-313-8109, email@example.com, or follow @carolyntweets on Twitter.
Inquirer staff writer Sarah Smith contributed to this article.