Matthew 7
>21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
Doing the Father's will is believing on Christ, not doing works.
John 6
37 All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.
38 For I came down from heaven, not to do mine own will, but the will of him that sent me.
39 And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day.
40 And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.
>22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
Those people were trusting in their works to save them then get cast into hell.
>23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
He can't say he never knew you if you used to be saved. He would have to say I used to know you.