The Jews used to say sarcastically that their hearts were "veiled", by which they meant that their hearts were so well protected against Islam that it could never touch them. This was their way of congratulating themselves on being staunch in their belief. The Holy Qur'an points out that this is not the firmness of faith, but a damnation, for they deny Islam which now is the true religion, and stick to a religion which has been abrogated. They, consequently, possess only "a little" faith ('Iman ایمان). Since a little faith is not acceptable, they turn out to be infidels.
The little “ faith" which they possessed pertained to the doctrines which are common to Islam and Judaism - for example, belief in Allah, or belief in the Day of Judgment. But they did not accept Sayyidna Muhammad ﷺ as a prophet, and the Holy Qur'an as the Word of Allah. So, their faith was not complete.
If the Verse describes 'the little faith' as 'Iman ایمان ، it does so only in the lexical sense, for 'Iman ایمان signifies total certitude, even if it pertains to certain things, and not to others. But from the point of view of the Shari'ah, such a partial faith cannot be described as 'Iman ایمان . The Shari` ah would accept as valid only that 'Iman ایمان which affirms with certitude each and everything that the Shari` ah requires one to affirm.
Muhammad bin Ishaq reported that Ibn `Abbas said that,
(And they say, "Our hearts are Ghulf."), means, "Our hearts are screened." Mujahid also said that,
(And they say, "Our hearts are Ghulf."), means, "They are covered."`Ikrimah said, "There is a stamp on them." Abu Al-`Aliyah said, "They do not comprehend." Mujahid and Qatadah said that Ibn `Abbas read the Ayah in a way that means, "Our hearts contain every type of knowledge and do not need the knowledge that you (O Muhammad) have." This is the opinion of `Ata' and Ibn `Abbas.
(Nay, Allah has cursed them for their disbelief) meaning, "Allah expelled them and deprived them of every type of righteousness." Qatadah said that the Ayah,
(So little is that which they believe.) means, "Only a few of them believe." Allah's statement,
(And they say, "Our hearts are Ghulf.") is similar to His statement,
(And they say: "Our hearts are under coverings (screened) from that to which you invite us) (41:5).
This is why Allah said here,
(Nay, Allah has cursed them for their disbelief, so little is that which they believe.) meaning, "It is not as they claim. Rather, their hearts are cursed and stamped," just as Allah said in Surat An-Nisa' (4:155),
(And of their saying: "Our hearts are wrapped (with coverings, i.e. we do not understand what the Messengers say) ـ nay, Allah has set a seal upon their hearts because of their disbelief, so they believe not but a little.)
There is a difference of opinion regarding the meaning of Allah's statement,
(So little is that which they believe.) and His statement,
(So they believe not except a few). Some scholars said that the Ayat indicate that a few of them would believe, or that their faith is minute, because they believe in Resurrection and in Allah's reward and punishment that Musa foretold. Yet, this faith will not benefit them since it is overshadowed by their disbelief in what Muhammad brought them. Some scholars said that the Jews did not actually believe in anything and that Allah said,
(So little is that which they believe), meaning, they do not believe. This meaning is similar to the Arabic expression, "Hardly have I seen anything like this," meaning, "I have never seen anything like this."