Pope St. John Paul II
“The tendency to celebrate the Eucharist only when there was an assembly of the faithful emerged in those years. According to the Council, although everything possible should be done to gather the faithful for the celebration, it is also true that, even if the priest is alone, the Eucharistic offering which he performs in the name of Christ has the effectiveness that comes from Christ and always obtains new graces for the Church. Therefore I, too, recommend to priests and to all the Christian people that they ask the Lord for a stronger faith in this value of the Eucharist.”
Source: Pope St. John Paul II, General Audience, June 9, 1993