For common sense as well as the teaching of Scripture shows that it is impossible for one who has not thoroughly cleansed himself from all the stains arising from evil to be admitted among the heavenly company. This is a thing which, though little in itself, is the beginning and foundation of great blessings. I call it little on account of the facility of the means of amendment. For what difficulty is there in this matter? Viz. to believe that God is everywhere, and that being in all things He is also present with those who call upon Him for His life-supporting power, and that, thus present, He does that which properly belongs to Him to do.
Now, the work properly belonging to the Divine energy is the salvation of those who need it; and this salvation proves effectual by means of the cleansing in the water; and he that has been so cleansed will participate in Purity; and true Purity is Deity. You see, then, how small a thing it is in its beginning, and how easily effected; I mean, faith and water; the first residing within the will, the latter being the nursery companion of the life of man. But as to the blessing which springs from these two things, oh! How great and how wonderful it is, that it should imply relationship with Deity itself!
Source: The Great Catechism (New Advent)