Muhammad, who according to Islam, was the greatest man to ever live, and the Seal of the Prophets, testified that he didn't even know whether God would send him to Heaven or Hell after he died, as we read:
Sahih al-Bukhari 3929
Narrated Um Al-Ala:
An Ansari woman who gave the pledge of allegiance to the Prophet (ﷺ) that the Ansar drew lots concerning the dwelling of the Emigrants. Uthman bin Maz'un was decided to dwell with them (i.e. Um Al-`Ala's family), Uthman fell ill and I nursed him till he died, and we covered him with his clothes.
Then the Prophet (ﷺ) came to us and I (addressing the dead body) said, "O Abu As-Sa'ib, may Allah's Mercy be on you! I bear witness that Allah has honored you." On that the Prophet (ﷺ) said, "How do you know that Allah has honored him?" I replied, "I do not know. May my father and my mother be sacrificed for you, O Allah's Messenger (ﷺ)! But who else is worthy of it (if not `Uthman)?" He said, "As to him, by Allah, death has overtaken him, and I hope the best for him. By Allah, though I am the Apostle of Allah, yet I do not know what Allah will do to me." By Allah, I will never assert the piety of anyone after him.
That made me sad, and when I slept I saw in a dream a flowing stream for `Uthman bin Maz'un. I went to Allah's Messenger (ﷺ) and told him of it. He remarked, "That symbolizes his (good) deeds."
This narration is also given in Sahih al-Bukhari 7018.
Above, a woman is grieving over the death of someone who she considered to be a very righteous man. She pronounces over his body that Allah has honored him. This leads Muhammad to tell her that even he himself did not know what Allah would do to him, implying that this man's fate, no matter how righteous he was, is far from known to them.
This sheds light on a passage in the Quran, in which Muhammad says the same thing:
Surah 46:9
9 Say, "I am not the first messenger ever sent, nor do I know what will happen to me or you. I only follow what is revealed to me. And I am only sent with a clear warning."
In the Quran, you'll notice that many verses are preceded by "Say, ...". This is because Muhammad is supposed to be repeating exactly what the "Angel Gabriel" told him. Therefore, above is Allah directly telling Muhammad to say that he does not know what will happen to himself, or anyone else.
Later, we also see that Muhammad prayed that he would not suffer the Punishment of the Grave, a sort of intermediate torment between death and the final judgment:
Sahih al-Bukhari 1372
Narrated Masruq:
Aisha said that a Jewess came to her and mentioned the punishment in the grave, saying to her, "May Allah protect you from the punishment of the grave." Aisha then asked Allah's Messenger (ﷺ) about the punishment of the grave. He said, "Yes, (there is) punishment in the grave." Aisha added, "After that I never saw Allah's Messenger (ﷺ) but seeking refuge with Allah from the punishment in the grave in every prayer he prayed."
See Sahih al-Bukhari 2822-23, 4707 for some other narrations attesting to the fact that Muhammad did this.
Therefore, according to Islam, Muhammad, despite being the greatest prophet of all, didn't know his fate. What does that say for you, Muslims? Does that give you any hope? If Muhammad had doubts about whether he was going to make it, you have less than no hope whatsoever. I encourage you to look at the Quran's Hell List for a good primer on what you're up against.
First, however, let's contrast the words of Muhammad with those of Jesus Christ, who said:
John 11:25-26
25 Jesus said unto her, I am the resurrection, and the life: he that believeth in me, though he were dead, yet shall he live:
26 And whosoever liveth and believeth in me shall never die. Believest thou this?
John 10:7-9, 27-29
7 Then said Jesus unto them again, Verily, verily, I say unto you, I am the door of the sheep.
8 All that ever came before me are thieves and robbers: but the sheep did not hear them.
9 I am the door: by me if any man enter in, he shall be saved, and shall go in and out, and find pasture.
27 My sheep hear my voice, and I know them, and they follow me:
28 And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.
29 My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand.
The contrast between the above passages and the words of Muhammad is immeasurable. They aren't even in the same realm of existence. One is scared, doubtful, and uncertain; the other is powerful, comforting, and clear. Jesus Christ knew what was going to happen after death, not only for himself, but for anyone who believes in Him.
That is the jewel that Christianity offers, which no other religion can offer, because no other religious leader had any idea what they were talking about. Jesus Christ declares that He has all power in Heaven and earth (Matthew 28:18), and can save all those who come to Him by faith (Hebrews 7:25), and He had the history, miracles, and prophecy to give what He said substance:
1 John 5:13
13 These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.