Unfortunately, many persons commit actual mortal sin. They realize that a particular act is gravely immoral, according to their own conscience, and yet they choose the act with full consent.
Can a person know about the Catholic Church, then outwardly reject Her, and yet still be saved? Yes, if the rejection is not an actual mortal sin, due to a lack of full knowledge. The person knows what the Church teaches, but his conscience errs by deciding that the Catholic faith is not the truest form of religion. This dispute about actual mortal sin and full knowledge is really about salvation. Who is saved? And are we judged by conscience, or simply by our compliance to objective moral norms?
Certainly supported in the Catholic teachings, tradition, and scripture:
Matthew 26
22 And they being very much troubled, began every one to say: Is it I, Lord?
23 But he answering, said: He that dippeth his hand with me in the dish, he shall betray me.
24 The Son of man indeed goeth, as it is written of him: but woe to that man by whom the Son of man shall be betrayed: it were better for him, if that man had not been born.
25 And Judas that betrayed him, answering, said: Is it I, Rabbi? He saith to him: Thou hast said it.
Matthew 5
17 Do not think that I am come to destroy the law, or the prophets. I am not come to destroy, but to fulfill. 18 For amen I say unto you, till heaven and earth pass, one jot, or one tittle shall not pass of the law, till all be fulfilled. 19 He therefore that shall break one of these least commandments, and shall so teach men, shall be called the least in the kingdom of heaven. But he that shall do and teach, he shall be called great in the kingdom of heaven.
20 For I tell you, that unless your justice abound more than that of the scribes and Pharisees, you shall not enter into the kingdom of heaven.
**
John 8**
31 Then Jesus said to those Jews, who believed him: If you continue in my word, you shall be my disciples indeed.
32 And you shall know the truth, and the truth shall make you free. …
34 Jesus answered them: Amen, amen I say unto you: that whosoever committeth sin, is the servant of sin…
39 They answered, and said to him: Abraham is our father. Jesus saith to them: If you be the children of Abraham, do the works of Abraham.
40 But now you seek to kill me, a man who have spoken the truth to you, which I have heard of God. This Abraham did not.
41 You do the works of your father. They said therefore to him: We are not born of fornication: we have one Father, even God.
42 Jesus therefore said to them: If God were your Father, you would indeed love me. For from God I proceeded, and came; for I came not of myself, but he sent me:
43 Why do you not know my speech? Because you cannot hear my word.
44 You are of your father the devil, and the desires of your father you will do. He was a murderer from the beginning, and he stood not in the truth; because truth is not in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father thereof.
45 But if I say the truth, you believe me not.
46 Which of you shall convince me of sin? If I say the truth to you, why do you not believe me?
47 He that is of God, heareth the words of God. Therefore you hear them not, because you are not of God.
Matthew 23
13 But woe to you scribes and Pharisees, hypocrites; because you shut the kingdom of heaven against men, for you yourselves do not enter in; and those that are going in, you suffer not to enter.
14 Woe to you scribes and Pharisees, hypocrites: because you devour the houses of widows, praying long prayers. For this you shall receive the greater judgment.
15 Woe to you scribes and Pharisees, hypocrites; because you go round about the sea and the land to make one proselyte; and when he is made, you make him the child of hell twofold more than yourselves.
16 Woe to you blind guides, that say, Whosoever shall swear by the temple, it is nothing; but he that shall swear by the gold of the temple, is a debtor.