It’s another form of negation, it holds the same meaning as “pas” but stronger.

Point is an outdated form of the negation. You can see “ne…point” as an old synonym for “ne…pas”. It has disappeared in modern French, but it is still understood when found in proverbs or literary works.

If there is pun in this sentence it is on raison and not at all on point.

If the use of point in the negation is considered old-fashioned nowadays, it was usual and common in 17th century writing (Les Pensées was published just after Pascal’s death in 1669). See this question Négation avec « point » for more about point.

The statement plays on the two meanings of the word raison. Les raisons are the reasons why, the causes and la raison is the ability of the mind to think, to make judgements.

This reflects Pascal’s religious belief. By this sentence he means we will get a better access to God by following our heart rather than by following our reason.

Point is like pas. It doesn’t add any particular emphasis to the sentence. In modern French, we’d say :

• Le cœur a ses raisons que la raison ne connaît pas.

When, as early as the 12th century, the negation ne had to be reinforced because it had been weakened in spoken French, several words came to play this role such as pas (step), mie (crumb), goutte (drop) and point (dot) :

• Je ne marche pas.
• Je ne mange mie.
• Je ne bois goutte.
• Je ne vois point.

The various adverbs reinforcing the negation ne lost their lexical meaning very early on in Old French and did not necessarilly match semantically the verbs they were used with as explained in Eauquidort’s remarks.

Pas has now superseded all the others and is the most widely used to reinforce or even to replace altogether ne. Point is considered literary or a regionalism. I remember an older guy from Ardèche telling me in conversation about the road police, Les gendarmes, j’en ai point vu.