How fares it with the happy dead?
For here the man is more and more;
But he forgets the days before
God shut the doorways of his head.


The days have vanish’d, tone and tint,
And yet perhaps the hoarding sense
Gives out at times (he knows not whence)
A little flash, a mystic hint;


And in the long harmonious years
(If Death so taste Lethean springs)
May some dim touch of earthly things
Surprise thee ranging with thy peers.


If such a dreamy touch should fall,
O, turn thee round, resolve the doubt;
My guardian angel will speak out
In that high place, and tell thee all.

This canto is dense and one has to guess at the meaning:

He compares life beyond death with human life in which the growing person, rather as Wordsworth imagines in the Immortality Ode, forgets all that happened before the infant head hardened. Only, there are hints and flashes, which Wordsworth called “fallings from us, vanishings,” that remind us of that other life. In the same way, he thinks, Hallam may have similar dim touches, remembrances of earthly life and his friendship with Tennyson. If his memory gives him no more, Tennyson’s guardian angel in heaven will give him the lowdown.

I wrote,dense, but I wonder about “ before/ God shut the doorways if the head.” It’s grand phrase for a physiological development in a baby, but how is the reader meant to find its precise meaning? Well by a bit of hard work on the text; but that detracts from any immediate impact, of what at first blush sounds like some mystical belief one ought to have known. Sometimes in Tennyson a grand phrase obscures rather than enlightens.