The Northern Renaissance is sometimes referred to as the prelude to the Protestant Reformation because it set the stage for the Reformation. It did so mainly in an intellectual sense.
As the humanists of the Northern Renaissance read writings from antiquity, their ideas changed in a very important way. First, they became acquainted with the way the early Christian church had been. They saw it as a much simpler religion, as opposed to the modern Catholic Church with its complex dogmas and its hierarchy. This led them to start thinking that the church should be simplified and made more like it had been in its earlier days.
Northern Renaissance humanism also believed in reform. Its thinkers thought that human beings could improve themselves and that they should try to do so. If human beings could improve themselves, then it made sense that human institutions like the Church could and should do so as well.
These sorts of ideas in the Northern Renaissance helped pave the way for the Reformation, which tried to improve the church and bring it closer to what it had been in its earlier days.