Have you got reservations on a train thal leaves CDG station at 2148 ? This is an RER local train across Paris to Massy. It gets to Massy at 2253, then the TGV leaves there 5 minutes later at 2258 to travel overnight to Lourdes, arrival 0646 next morning.
This train starts out from Paris Montparnasse station in the centre of Paris but the connection in the southern suburbs at Massy is the most effective one from CDG.
The RER is a frequent local service, if you leave CDG earlier you will have a bit more time for the connection.