Have you read this in Scripture?
Luke 12:10 And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven.
Obviously, Jesus, Himself, explains that even those who attack Jesus Christ (i.e. Son of man), may be forgiven. Therefore, those who do the will of God and yet do not know Christ, as, for instance, the Jews, may, in the end, be forgiven.
Acts 10:34 Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons: 35 But in every nation he that feareth him, and worketh righteousness, is accepted with him.
Jesus Christ is explicit, that even some who reject Him, will be saved. Scripture is also explicit, that it is Jesus who judges who is saved.
Romans 2:16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
As the parable is clear to explain, it is not the one who denies Christ by word which will be condemned. But the one who does not do His will:
Matthew 21:28 But what think ye? A certain man had two sons; and he came to the first, and said, Son, go work to day in my vineyard.
29 He answered and said, I will not: but afterward he repented, and went.
30 And he came to the second, and said likewise. And he answered and said, I go, sir: and went not.
31 Whether of them twain did the will of his father? They say unto him, The first. Jesus saith unto them, Verily I say unto you, That the publicans and the harlots go into the kingdom of God before you.
32 For John came unto you in the way of righteousness, and ye believed him not: but the publicans and the harlots believed him: and ye, when ye had seen it, repented not afterward, that ye might believe him.
Therefore, God judges a man by his works. Not by his faith alone.
Coming to Christ means coming to His Sacraments which are administered by His Church. Those who do not come to His Church do not come to Christ and must await the Judgment in order to receive the grace of eternal life. All who deny the Sacraments put themselves under the law and are judged according to the law.
But Judgment comes first to the House of God, through the Sacraments. Wherein, those who do the Will of God, walk amongst the Saints of God, in this life.
1 Peter 4:17 For the time is come that judgment must begin at the house of God: and if it first begin at us, what shall the end be of them that obey not the gospel of God?
Even though atheists and Jews and Muslims might be saved, it is by no means going to be easy for them to be so.
Frankly speaking, it won't be easy for any ANTI-Catholics to be saved. The Catholic Church (and the other Sacramental, Apostolic Churches, such as the Orthodox) affords the greatest assistance for salvation, in the Sacraments of Jesus Christ.
Mark 9:40 For he that is not against us is on our part.
Jesus is explaining that many people will inadvertently reject Christ even when Christ is the One whom they seek and await:
John 5:38 And ye have not his word abiding in you: for whom he hath sent, him ye believe not. 39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me. 40 And ye will not come to me, that ye might have life.
John 16:2 They shall put you out of the synagogues: yea, the time cometh, that whosoever killeth you will think that he doeth God service.
Unlike you or I, God can read the heart.
Have you not read in Scripture that:
Matthew 21:31 Whether of them twain did the will of his father? They say unto him, The first. Jesus saith unto them, Verily I say unto you, That the publicans and the harlots go into the kingdom of God before you.
This is why Pope Francis said,
"Francis based his homily on the message of Christ to his disciples taken from the Gospel of Mark. Francis delivered his message by sharing a story of a Catholic who asked a priest if atheists were saved by Christ.
"They complain," Francis said, "If he is not one of us, he cannot do good. If he is not of our party, he cannot do good." He explained that Jesus corrected them, "Do not hinder him, he says, let him do good."
The disciples, Pope Francis explained, "were a little intolerant," closed off by the idea of possessing the truth, convinced that "those who do not have the truth, cannot do good." "This was wrong... Jesus broadens the horizon." Pope Francis said, "The root of this possibility of doing good - that we all have - is in creation."
"Even them, everyone, we all have the duty to do good, Pope Francis said on Vatican Radio.
"Just do good" was his challenge, "and we'll find a meeting point."
Francis explained himself, "The Lord created us in His image and likeness, and we are the image of the Lord, and He does good and all of us have this commandment at heart, do good and do not do evil. All of us. 'But, Father, this is not Catholic! He cannot do good.' Yes, he can... "The Lord has redeemed all of us, all of us, with the Blood of Christ, all of us, not just Catholics. Everyone! 'Father, the atheists?' Even the atheists. Everyone!" We must meet one another doing good. 'But I don't believe, Father, I am an atheist!' But do good: we will meet one another there."
Now, Protestants will object that we are saved by "faith alone."
But Scripture proves them wrong. Because Scripture says that faith alone is a gong and a clanging cymbal. Whereas, love (i.e. charity) is the greatest of these. And good works, are expressions of love:
1 Corinthians 13:1 Though I speak with the tongues of men and of angels, and have not charity, I am become as sounding brass, or a tinkling cymbal. 2 And though I have the gift of prophecy, and understand all mysteries, and all knowledge; and though I have all faith, so that I could remove mountains, and have not charity, I am nothing. 3 And though I bestow all my goods to feed the poor, and though I give my body to be burned, and have not charity, it profiteth me nothing. 4 Charity suffereth long, and is kind; charity envieth not; charity vaunteth not itself, is not puffed up, 5 Doth not behave itself unseemly, seeketh not her own, is not easily provoked, thinketh no evil; 6 Rejoiceth not in iniquity, but rejoiceth in the truth; 7 Beareth all things, believeth all things, hopeth all things, endureth all things. 8 Charity never faileth: but whether there be prophecies, they shall fail; whether there be tongues, they shall cease; whether there be knowledge, it shall vanish away. 9 For we know in part, and we prophesy in part. 10 But when that which is perfect is come, then that which is in part shall be done away. 11 When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things. 12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.
13 And now abideth faith, hope, charity, these three; but the greatest of these is charity.
Galatians 5:6 For in Jesus Christ neither circumcision availeth any thing, nor uncircumcision; but faith which worketh by love.
Faith alone, is dead.