The phrase "momentous step" means a very important and significant action or decision that could have a big impact or bring about significant changes. Full definition
Of course, most of The Walk is pure prelude, 80 minutes of time killing until Petit takes that first momentous step off the edge of the south tower. (film.avclub.com)
A quasi-religious order of black - robed women move their secret breeding program one momentous step closer to creating a god - child they call the Kwisatz Haderach. (bookbrowse.com)
In June, the Scottish Episcopal Church (SEC) described its move to allow gay weddings in its buildings as «a momentous step». (premierchristianity.com)