If there’s anything worse than a linguistic rabbit hole, it’s a theological rabbit hole.
Let us all Thy grace receive.
(Let) (us all) (Thy grace) (receive)
(Let) (us) (all Thy grace) (receive)
Linguistically, there’s no way to decide in this case. Both are grammatical and usual/natural. In both, the word all can be omitted, perhaps with a change of emphasis but not of basic meaning. To the extent that I’d ever thought about it, I had always assumed the first reading.
Theologically (and I Am Not A Theologian), the first question is whether humans can receive all God’s grace. If not, then that reading must be discarded, even if it is grammatically possible.
I texted a member of the choir who is a senior lay reader, and he said, yes, Charles Wesley was into “Christian perfection”, which Wikipedia explains:
Christian perfection is the name given to various teachings within Christianity that describe the process of achieving spiritual maturity or perfection. The ultimate goal of this process is union with God characterized by pure love of God and other people as well as personal holiness or sanctification.
This doctrine has its supporters and opponents. Personally (and I Am Not A Theologian), humans simply cannot be perfect in this life, though some people get closer to it than others.
Further, he said that Wesley’s original had “Let us all Thy life receive”. Wikipedia discusses a number of changes which have been made to the original over the years, including this one. Apparently, there is a difference between God’s life and God’s grace, which made some people want to change it.
Jumping back to linguistics, it is sometimes possible to discern a writer’s intention by looking at how he (or she) uses the same word elsewhere in the same text or in other similar writings.
Crucially, Wesley writes in his original second verse, which is omitted in every hymn book I’ve ever seen this hymn in:
Let us all in Thee inherit.
This can’t be:
(Let) (us all) (in Thee) (inherit)
because we can’t inherit “in thee”; we must inherit something, in this case “all in Thee”.
Therefore, it can only be:
(Let) (us) (all in Thee) (inherit).
But that doesn’t necessarily mean that the first line under discussion has (or has to have) the second reading. It is perfectly possible for similar lines in a hymn to have different interpretations.
Maybe Wesley didn’t notice. Maybe he intended the ambiguity (my fellow chorister wrote “That’s poetry for you”). On the websites I looked at (some more, some less thoroughly) while thinking about this, no-one else seems to discuss it.
I guess I’ll never know. It may not matter. Salvation isn’t what Charles Wesley says it is; it’s what God says it is.
PS I realised later that there’s nothing in the hymn about when any of this will take place, but the last verse contains “Till in heav’n we take our place”, which suggests that everything before it is not in heaven.