St. Thomas Aquinas
“Every medicine does not suit every stage of sickness; because the tonic given to those who are recovering from fever would be hurtful to them if given while yet in their feverish condition. So likewise Baptism and Penance are as purgative medicines, given to take away the fever of sin; whereas this sacrament (Eucharist) is a medicine given to strengthen, and it ought not to be given except to them who are quit of sin.”
Source: St. Thomas Aquinas, Summa Theologica