John Calvin Commentary Psalms 138:3

"In the day that I called thou answeredst me, Thou didst encourage me with strength in my soul." — Psalms 138:3 (ASV)

In the day when I cried to you, etc. Frequently God anticipates our prayers and surprises us, as it were, while we are sleeping. But commonly he stirs us up to prayer by the influence of his Spirit, and he does this to illustrate his goodness further, as we find that he crowns our prayers with success.

David rightly infers that his escape from danger could not have been merely coincidental, as it plainly appeared that God had answered him. This, then, is one noteworthy point: that our prayers more clearly reveal his goodness to us.

Some add a conjunction in the second part of the verse: You have increased me, and in my soul is strength. But this is unnecessary, since the words read well enough as they are, whether we translate the passage as I have done above, or as: You have multiplied, or increased, me with strength in my soul. The meaning is that from a weak and afflicted state, he had received fresh strength to his spirit. Or some may, perhaps, prefer to interpret it this way: You have multiplied—that is, blessed me, from which came strength in my soul.