This may be viewed as the climax of the novel in that it represents Charles Ryder's emotional acceptance of the Catholic faith.
Ryder's experience differs from that of Evelyn Waugh who noted that he entered the Catholic Church on firm intellectual conviction but with little emotion. This assessment was supported by Father D'Arcy, the priest who instructed him: [Waugh] had convinced himself very unsentimentally - with only an intellectual passion, of the truth of the Catholic faith, and that in it he must save his soul.
Source: Evelyn Waugh: A BIOGRAPHY by Selina Hastings (Sinclair-Stevenson, 1994) p.227.
The reference to the veil of the temple comes from the Gospels of Matthew (27:51), Luke (23:45) and Mark (15:38); it describes what happened at the moment of Jesus' death on the cross. The Matthew version reads:
And behold, the veil of the temple was rent in twain from the top to the bottom, and the earth did quake and the rocks rent.
The events at Lord Marchmain's deathbed are therefore of momentous significance to Charles.