Old Hundredth

I've decided to start reading the hymns in my battered old Salvation Army Songbook (1955 edition) as part of my daily devotions, starting from the beginning. Today's hymn was #2, "Old Hundredth."

This verse caught my attention:

For why? The Lord our God is good,
His mercy is for ever sure;
His truth at all times firmly stood,
And shall from age to age endure.