I think the section of this amazing poem you want to refer to is the last part, when Wordsworth addresses his sister and the power of memory is mentioned in what he says to her. Wordsworth tells his sister to remember this present moment and the beauty of nature in future when she may be feeling sad, afraid, alone or upset. Remembering the beauty of nature and how it has impacted Wordsworth himself will enable her to overcome those feelings. Note what the poem says:
Therefore let the moon
Shine one thee in thy solitary walk;
And let the misty mountain-winds be free
To blow against thee: and, in after years,
When these wild ecstasies shall be matured
Into a sober pleasure, when they mind
Shall be a mansion for all lovely forms,
Thy memory be as a dwelling place
For all sweet sounds and harmonies; oh! then,
If solitude, or fear, or pain, or grief,
Should be thy portion, with what healing thoughts
Of tender joy wilt thou remember me...
Memory then is explicity associated with "healing thoughts" and is described as a "dwelling place / For all sweet sounds and harmonies." I am not too sure that memory can be described as a source of salvation--it seems a little too strong to me--but it is clear that Wordsworth presents memory as something that we can use to overcome our present challenges through focusing on the important lessons that we have learnt in the past. Our minds can become a "mansion for all lovely forms," and through this we can overcome whatever fears or doubts we are facing in the present.