Is uttering the Shahaadatayn sufficient to enter Paradise?

Islam is not just uttering the Shahaadatayn only, rather it
is essential to fulfil the conditions of this twin declaration of faith so
that the one who utters them will truly be a Muslim. The pillars of Islam
are believing, uttering and acting. 

It was narrated that Ubaadah ibn al-Saamit said: The
Messenger of Allaah (peace and blessings of Allaah be upon him) said:
“Whoever says ‘I bear witness that there is no god except Allaah alone [with
no partner or associate] and that Muhammad is His slave and Messenger, and
that ‘Eesa is the slave of Allaah, the son of His maidservant, a Word which
He bestowed upon Maryam and a Spirit created by Him, and that Paradise is
true and that Hell is true, Allaah will admit him through whichever of the
eight gates of Paradise he wants.”

Narrated by al-Bukhaari (3252) and Muslim (28).   

Shaykh ‘Abd al-Rahmaan ibn Hasan ibn Muhammad ibn ‘Abd
al-Wahhaab (may Allaah have mercy on him) said: 

The words “Whoever says ‘I bear witness that there is no god
except Allaah alone” mean, the one who speaks these words, knowing what they
mean and acting upon them inwardly and outwardly, because with regard to the
Shahaadatayn it is essential that one know and be certain and act upon them
as Allaah says (interpretation of the meaning): “So know (O Muhammad
صلى الله عليه وسلم) that Laa ilaaha
illAllaah (none has the right to be worshipped but Allaah)” [Muhammad 47:19]. 

And Allaah says (interpretation of the meaning):  “except
for those who bear witness to the truth knowingly (i.e. believed in the
Oneness of Allaah, and obeyed His Orders), and they know (the facts about
the Oneness of Allaah)” [al-Zukhruf 43:86]. As for uttering the
words without knowing the meaning or believing with certainty or acting upon
them by disavowing shirk and being sincere in word and deed, and saying in
one’s heart and on one’s lips and acting in one’s heart and with one’s
outward faculties, that is of no benefit, according to scholarly consensus.
 

Al-Qurtubi said in al-Mufhim ‘ala Saheeh Muslim:
Chapter: it is not sufficient merely to speak the Shahaadatayn, rather it is
essential to believe it with certainty in one’s heart. This draws attention
to the flawed argument of the extreme Murji’ah who say that merely uttering
the Shahaadatayn is sufficient to make one a believer. The ahaadeeth on this
subject indicate that this is wrong; rather it is a view that is well known
to be wrong according to sharee’ah, because it implies a justification of
hypocrisy, and judging hypocrites as true believers, which is completely
false. End quote. 

This hadeeth also indicates this, in the words “whoever bears
witness” because bearing witness can only be valid if it is based on
knowledge, certainty, sincerity and honesty. 

Fath al-Baari (p. 36). 

There are seven conditions of bearing witness that there is
no god but Allaah, and it does not benefit the one who says it unless he
fulfils all of them. They are, in brief: 

1-   
Knowledge as opposed to
ignorance

2-   
Certainty as opposed to doubt

3-   
Acceptance as opposed to
rejection

4-   
Obedience as opposed to
disobedience

5-   
Sincerity as opposed to shirk

6-   
Honesty as opposed to lying

7-   
Love as opposed to its
opposite, which is hate. 

The conditions of bearing witness that Muhammad is the
Messenger of Allaah are the same as the conditions of bearing witness that
there is no god but Allaah, which have been mentioned with evidence in the
answers to questions no. 9104 and
12295. 

And Allaah knows best.