What's the summary of John Donne's sonnet VIII?

Great question!  Probably you mean sonnet VIII (or 8) from
Donne's famous collection titled Holy Sonnets. Here is the poem
itself:

If faithfull soules be alike glorifi'd
As
Angels, then my fathers soul doth see,
And adds this even to full felecitie,

That valiantly I hels wide mouth o'stride:
But if our mindes to these
soules be descry'd
By circumstances, and by signes that be
Apparent in us,
not immediately,
How shall my mindes white truth by them be try'd?
They see
idolatrous lovers weepe and mourne,
And vile blasphemous Conjurers to call

On Jesus name, and Pharisaicall
Dissemblers feigne devotion. Then turne

O pensive soule, to God, for he knows best
Thy true griefe, for he put it
in my breast.

Here is a summary of the meaning of the
poem:

If faithfull soules be alike glorifi'd

As Angels, [if faithful souls are
glorified in heaven as angels are glorified]

then my
fathers soul doth see,
And adds this even to full
felecitie,
That valiantly I hels wide mouth o'stride:

[then the soul of my dead father, who is now in
heaven, can see that I have so far managed to avoid being pulled into the mouth of hell. His
perception that I am not damned to hell adds to his complete and eternal happiness, or
felicity]

But if our mindes to these soules be
descry'd
By circumstances, and by signes that be

Apparent in us, not
immediately,

[However,
if the glorified souls in heaven can look into the minds of humans who are still alive on earth,
and if they can perceive the states of our minds by various pieces of evidence
("signs")]

How shall my mindes
white truth by them be try'd?

[How, then, will my
mind be perceived by them?]

They see idolatrous lovers
weepe and mourne,
And vile blasphemous Conjurers to call
On Jesus name, and
Pharisaicall
Dissemblers feigne
devotion.

[After all, the glorified souls in heaven
can look down on earth and see all kinds of religious hypocrisy, including the hypocrisy of those
who publically profess their Christianity but are not sincere
Christians.]

Then turne
O
pensive soule, to God, for he knows best
Thy true griefe, for he put it in my breast.

[Therefore, I urge my soul to turn its attentions to
God.  God is in the best position to know the grief I feel, because he put that grief into my
heart, perhaps as a way of convincing me of my sinfulness and thus of my need for
him.]