A “high and holy lay” is a sacred song, what we would call a hymn. Even with that tidbit of knowledge it takes a little thinking to make sense of that first verse.
In vain in high and holy lays
My soul her grateful voice would raise,
For who can sing the worthy praise of the
Wonderful love of Jesus?
It is impossible for human voices, even singing the holiest songs they can compose, to praise the love of Christ as much as it deserves. All our efforts are “in vain.” That’s what it means.
How can it have taken so long for me to figure it out when the scriptures are full of the same thought?
Praise Jehovah. Oh give thanks unto Jehovah; for he is good; For his lovingkindness [endures] for ever. Who can utter the mighty acts of Jehovah, Or show forth all his praise? Psalm 106:1,2.
You have multiplied, O LORD my God, your wondrous deeds and your thoughts toward us; none can compare with you! I will proclaim and tell of them, yet they are more than can be told. Psalm 40:5.
My mouth will tell of your righteous acts, of your deeds of salvation all the day, for their number is past my knowledge. Psalm 71:15
Oh, the depth of the riches and wisdom and knowledge of God! How unsearchable are his judgments and how inscrutable his ways! “For who has known the mind of the Lord, or who has been his counselor?” “Or who has given a gift to him that he might be repaid?” Rom 11:33-35.
As you can see, it isn’t just the love of God we cannot speak adequately of, but also His wisdom, His righteousness, and His mighty works. If ever there was a hymn of humility it is this one. We cannot even begin to fully comprehend any of the Godhead and thus we cannot praise as they deserve. We do so “in vain,” yet our gratitude continues to compel us to try, and so we do with songs like this one. It may be “high and holy” but it is not equal to the task.
Yet now that I know what I am singing, I can hardly wait to sing it again.
[That you] may have strength to comprehend with all the saints what is the breadth and length and height and depth, and to know the love of Christ that surpasses knowledge, that you may be filled with all the fullness of God. Eph 3:18, 19.