A Christian is asking what is written on the gate of Paradise?

Firstly: 

Paradise does not have only one gate, rather it has many
gates. Allaah says (interpretation of the meaning): 

“And those who kept their duty to their Lord (Al-Muttaqoon
– the pious) will be led to Paradise in groups till when they reach it, and
its gates will be opened (before their arrival for their reception) and its
keepers will say: Salaamun ‘Alaykum (peace be upon you)! You have done well,
so enter here to abide therein”

[al-Zumar 39:73]

There is also evidence
narrated in the Sunnah which points to the number of these gates. For
example, the Prophet (peace and blessings of Allaah be upon him) said:
“In Paradise there are eight gates, among which is a gate called al-Rayyaan,
through which no one will enter except those who fast.” Narrated by
al-Bukhaari, 3257; Muslim, 1125. This hadeeth indicates that the number of
gates is eight. 

Secondly: 

There are some reports which speak of what is written on the
gate of Paradise. The strongest report that we have found is that which was
narrated by al-Tabaraani and al-Bayhaqi from Abu Umaamah (may Allaah be
pleased with him), that the Prophet (peace and blessings of Allaah be
upon him) said: “A man entered Paradise and saw written on its gate ‘Charity
receives a tenfold reward and a loan an eighteenfold reward.’” 

Classed as hasan by al-Albaani. See Saheeh al-Targheeb
wa’l-Tarheeb, 1/537. 

Although some scholars
were inclined to accept the attribution of this hadeeth to the Prophet
(peace and blessings of Allaah be upon him), others – including Ibn
al-Jawzi, al-‘Iraaqi, al-Manaawi and others – classed it as da’eef (weak),
because the narrators who transmitted it from the Prophet (peace and
blessings of Allaah be upon him) include a narrator who is very weak. 

There is another report which says: “Written on the gates of
Paradise is: ‘Laa ilaaha ill-Allaah Muhammad Rasool-Allaah ‘Ali akhoo
rasool-Illaah (There is no god but Allaah, Muhammad is the Messenger of
Allaah, ‘Ali is the brother of the Messenger of Allaah).’ This was written
two thousand years before the creation of the heavens and the earth.” 

But this is a false report which cannot be attributed to the
Messenger of Allaah (peace and blessings of Allaah be upon him). See
al-Silsilah al-Da’eefah, vol. 10, no. 4901. 

We should also note that
speaking about this topic or similar topics that have to do with Paradise,
Hell or other matters of the unseen cannot be accepted as correct unless it
is based on revelation (wahy) that is proven to be from Allaah, on the lips
of His Messenger. 

And we have to ask ourselves: what is the practical benefit
of knowing such things? What good things will we be missing out on if we do
not know this? 

The question which we should really be asking is: How can we
get to those gates? How can they be opened for us? How can we enter them? 

This is the decisive question which people should concern
themselves with. 

In truth there is only one way to reach Paradise: the
straight path in which there is no crookedness. Allaah says (interpretation
of the meaning): 

“And verily, this is My straight path, so follow it, and
follow not (other) paths, for they will separate you away from His path.
This He has ordained for you that you may become Al‑Muttaqoon (the pious)”

[al-An’aam 6:153]

The first step on that straight path is believing that there
is no god but Allaah and that Muhammad is the Messenger of Allaah, believing
that Muhammad (peace and blessings of Allaah be upon him) is a Prophet
and Messenger from Allaah. This is the matter concerning which Allaah took
the covenant and pledge from the Prophets before they were sent. Allaah says
(interpretation of the meaning):  

“And (remember) when
Allaah took the Covenant of the Prophets, saying: ‘Take whatever I gave you
from the Book and Hikmah (understanding of the Laws of Allaah), and
afterwards there will come to you a Messenger (Muhammad) confirming what is
with you; you must, then, believe in him and help him.’ Allaah said: ‘Do you
agree (to it) and will you take up My Covenant (which I conclude with you)?’
They said: ‘We agree.’ He said: ‘Then bear witness; and I am with you among
the witnesses (for this).’”

[Aal ‘Imraan 3:81]

Allaah called all of those who were given a book before him
to believe in this final Prophet and His plain Book. Allaah says
(interpretation of the meaning): 

“O people of the
Scripture (Jews and Christians)! Now has come to you Our Messenger
(Muhammad) explaining to you much of that which you used to hide from the
Scripture and pass over (i.e. leaving out without explaining) much. Indeed,
there has come to you from Allaah a light (Prophet Muhammad) and a plain
Book (this Qur’aan).

16. Wherewith Allaah
guides all those who seek His Good Pleasure to ways of peace, and He brings
them out of darkness by His Will unto light and guides them to the Straight
Way (Islamic Monotheism)”

[al-Maa’idah 5:15-16] 

Allaah tells us that His proof is established against the
People of the Book and others by means of this final Prophet. Allaah says
(interpretation of the meaning): 

“O people of the
Scripture (Jews and Christians)! Now has come to you Our Messenger
(Muhammad) making (things) clear unto you, after a break in (the series of)
Messengers, lest you say: ‘There came unto us no bringer of glad tidings and
no warner.’ But now has come unto you a bringer of glad tidings and a
warner. And Allaah is Able to do all things”

[al-Maa’idah 5:19] 

After he has come, no one can enter Paradise unless they
follow him and obey him. 

It was narrated that Jaabir ibn ‘Abd-Allaah (may Allaah be
pleased with him) said: Some angels came to the Prophet while he wassleeping. Some of them said, “He is sleeping.” Otherssaid, “His eyes are sleeping but his heart is awake.” Then they said,
“There is an example for this companionof yours.”
One of them said, “Then set forth an examplefor
him.” Some of them said, “He is sleeping.” Theothers
said, “His eyes are sleeping but his heart is awake.” Then they said, “His
example is that of a manwho has built a house and
then offered therein abanquet and sent a messenger
to invite thepeople. So whoever accepted the
invitation of themessenger, entered the house and
ate of the banquet, andwhoever did not accept the
invitation of the messenger, didnot enter the house,
nor did he eat of the banquet.” Then the angels
said, “Explain this example to him sothat he may
understand it.” Some of them said, “He issleeping.”
The others said, “His eyes are sleeping buthis heart
is awake.” And then they said, “Thehouse stands for
Paradise and the caller isMuhammad; and whoever
obeys Muhammad, obeysAllaah; and whoever disobeys
Muhammad, disobeysAllaah. Muhammad separated the
people (i.e., throughhis message, the good are
distinguished from the badand the believers from the
disbelievers).”  

Saheeh al-Bukhaari, 2861. 

According to a report narrated by al-Daarimi: Rabee’ah
al-Jurashi said: Some (angels) came to the Prophet (peace and blessings
of Allaah be upon him) and it was said to him: “Let your eyes sleep and let
your ears hear and let your heart understand.” He said: “My eyes slept but
my ears heard and my heart understood.” He said: “It was said to me: A
master built a house, then he prepared a banquet and sent a caller (to
invite people). Whoever responded to the caller entered the house and ate of
the banquet and the master was pleased with him, and whoever did not respond
to the caller did not enter the house or eat of the banquet, and the master
was angry with him.” Allaah is the master, Muhammad is the caller, the house
is Islam and the banquet is Paradise.”  

Sunan al-Daarimi, 11. 

If you want to study some of the foretelling of the coming of
the Prophet Muhammad (peace and blessings of Allaah be upon him) that
is mentioned in the Torah and Gospel (the Bible), you can refer to the book
Izhar al-Haqq by Shaykh Rahmatullah al-Hindi. 

May Allaah guide us and
you to His Straight Path, the path of those on whom Allaah has bestowed His
grace, not the path of those who have earned His anger nor of those who went
astray.