I don’t need to show any documents; it’s a perennial tenement of the Church handed down to us and is something I’ve myself heard from the Pulpit of my own Deanery. Those suggesting this somehow changed as a result of Vatican II are probably adherents of the false interpretation of Vatican II known as the Bologna School. To suggest on a Catholic website that one can show up basically at any time and still make their obligation, let alone receive communion, is rather worrying. Nothing really “went out with Vatican II;” rather Vatican II is the scapegoat for the abuses we have currently for which there is no basis, some of the worst being the claim that teachings actually changed following Vatican II when all Vatican II did was reaffirm what the Church had already been doing! Chant is still given “price of place” in the Mass and the rubrics still assume the Mass is going to be offered Ad Orientem. Likewise, one makes their Sunday obligation when they’ve arrived to Mass before the Offertory, unless it can be pointed to where the documents of Vatican II suggest this should no longer the case. There is zero to be scrupulous about when it’s so clearly defined; scruples might only come when one can’t get a clear answer. I myself have arrived after the Offertory and therefore mentioned it in my next confession, having refrained from the Eucharist. Life goes on but let’s not mince words and pretend it’s NOT a mortal sin!