Ah, there's a "but" when it comes to the end of the world, isn't there? This is called the "turn" in the sonnet, when the poem shifts topics (see "Form and Meter" for more).
We hear the speaker asking God not to have the angels wake up all these deceased people just yet.
We want to say: but you're the one who ordered him to wake them up! This is like hitting the snooze button on the Apocalypse. Those poor dead people: they are very tired and need their rest. ("Sleep" here is a metaphor for the time between death and resurrection.)
The real reason for this delay comes out in the second half of the line: it's all about "me."
The speaker wants some unspecified period of time to mourn for all the dead people.
Count us skeptical on that one. Do we really think he wants to mourn some long-dead people he has never met?
For, if above all these, my sins abound, 'Tis late to ask abundance of thy grace, When we are there; […]
The speaker worries that his own sins might be greater than those of all the "sleeping" people.
If his sins are really that bad, it will be too late for him to be forgiven on Judgment Day. He needs to start working toward forgiveness now.
His sins "abound" like sand abounds in a desert. "Abound" here means "to have a lot of" or "to be well-stocked with."
It seems like the speaker has a lot of sins, above and beyond the rest of sleeping humanity.
In a lot of older Christian poetry and literature, the way that you show humility is to say that you are the biggest sinner of them all; the worst of the worst. That's what the speaker is doing here.
Judgment Day is described as a place, "there."
The speaker is still talking to God at this point, and he is anticipating the time when he will have to stand before God and account for all his sins.
If he has repented enough, God will show His "grace" through forgiveness. In Christian thinking, God's grace is "abundant" enough that anyone who asks earnestly for forgiveness will be granted it.
[…] here on this lowly ground, Teach me how to repent; for that's as good As if thou hadst seal'd my pardon, with thy blood.
In the middle of line 12, the poem shifts topics one more time. The shift is marked by the contrast between "there" and "here." The speaker leaves off with his grand imaginings of resurrection and returns to the present moment.
The speaker asks God to teach him how to "repent" or ask for forgiveness (line 13). If God teaches him how to repent, the result would be the same as if God had sealed an official document of pardon with his own blood.
(Back in the day, people used a wax "seal" to make documents official. It was kind of like a signature. The speaker suggests that God's blood is like his personal seal.)
Remember when your mom told you there was a right way and a wrong way to say you're sorry? That seems to hold true in religious matters, as the speaker makes it sound like asking for forgiveness is a difficult task that requires a great teacher.
Coming back to the present and the earth seems like "lowly ground" compared to the standing in front of God at the Apocalypse.
But wait: like many of Donne's poems, this one has a twist at the end.
The speaker compares learning to repent to having a pardon sealed in blood. The pardon would absolve him of his crimes.
But the word blood might remind you of another story – the crucifixion of Jesus. According to Christian thought, Jesus died for the sins of mankind. We are meant to think of Jesus' blood as this seal of pardon.
The speaker shows his reverence for God even as he asks for God's help, and the poem itself is an act of repentance.