They are heretics who claim they live a “good life” but do not enter by the true door which is Christ and His true Church

“I am the Door * saith the Lord, by Me if any man enter in, he shall be saved, and shall find pasture.” Alleluia.
(Canticle of Zacharias: Luke 1:68-79)

John 10:1-10
“At that time: Jesus said unto the Pharisees: Amen, amen, I say unto you, he that entereth not by the door into the sheep-fold, but climbeth up some other way, the same is a thief and a robber but he that entereth by the door is the shepherd of the sheep..”

     “In the words of the Gospel which are this day read, the Lord has spoken unto us in similitudes, touching His flock, and the Door whereby entry is made into their fold. The Pagans therefore may say, "We have good lives," but if they enter not in the Door, what doth that profit them whereof they make their boast? Good life is profitable to a man if it lead unto life everlasting, but if he does not have life everlasting, what shall his good life profit him? Neither indeed can it be truly said that they live good lives, who are either so blinded as not to know, or so puffed up as to despise, the end of a good life. And no man can have a true and certain hope of life everlasting, unless he know the true Life, Which is Christ, and enter in by that Door into the sheepfold.

     “There are many such, who try to persuade men to live good lives but not to be Christians. These are they who would fain "climb up some other way," "for to kill and to destroy," and are not as the Good Shepherd, Who is come to keep and to save. There have been philosophers who have treated many subtle questions of right and wrong, who have been the authors of many distinctions and definitions, who have completed many exceedingly clever arguments, who have filled many books, and have proclaimed their own wisdom with braying trumpets. These dared to say to men: "Follow us embrace our school of thought, and you will find therein the secret of an happy life." But these were not of them who enter in by the Door; they came not but for to steal, and to kill, and to destroy.

     “Touching these, what shall I say? Behold, the Pharisees themselves read of Christ, and therefore talked of Christ they looked for His coming, and when He came, they knew Him not. They boasted that they themselves were among the Seers, that is, of the wise ones, and they denied Christ, and entered not in by the Door. Therefore they, if they led away any, led them away only to kill and to destroy, not to free them. So much for them. Now let us see if all they who boast the name of Christian enter in by the Door. Some there are, and their number cannot be reckoned, who not only boast that they themselves are among the Seers, but would fain appear as though their hearts were enlightened by Christ but they are heretics.“

- Homily by St. Augustine, Bishop of Hippo

45th Tract on John