T'hillim / Psalm 40 begins with the words "For the conductor, of David a song."
This line is either omitted from Christian translations as in the King James Translation (KJV) or is given almost as a header (it is not numbered) as in the NIV translation. Ergo it is easy for the missionary to ignore (or miss) that the psalms were primarily written by David about his own life -- and expressing universal emotions.
Who among us who loves G-d does not delight in doing His will?
The very fact that Jews live by his 613 mitzvot shows that we delight in following His words, His will. . .
The use of this as a "prophecy" about Jesus is truly remarkable when one considers that in the referenced lines alone:
Yet again the list maker has taken a non-prophecy (David saying "You desired neither sacrifice nor meal offering; You dug ears for me; a burnt offering or a sin offering You did not request. Then I said, "Behold I have come," with a scroll of a book written for me. O G-d, I desired to do Your will and [to have] וְ֝ת֥וֹרָתְךָ֗ / Your Torah within my innards." T'hillim / Psalm 40:7-9 (6-8 in Christian translations).
If anything this psalm and these particular verses within it prove that Jesus was NOT the messiah. Interesting that the list maker chose them as some sort of proof that Jesus "fulfilled 365 prophecies" in the T'nach!
Lastly, in this psalm the subject (David and almost all of us) speaks about his being a sinner. Jesus was, supposedly, without sin. How, then, is this psalm about Jesus? T'hillim / Psalm 40:13 "my עֲ֭וֹנֹתַי / avon / iniquities have overtaken me and I could not see [them because] they are more numerous than the hairs of my head"
An avon / עוון avon (the sins the subject admits to being guilty of doing), refers to willful, knowing transgression of G-d's law where one's desires get the upper hand.
If Jesus is sinless do the missionaries admit he was guilty of willfully breaking G-d's laws? Well then -- how does that equal him being the perfect "without sin" sacrifice? To understand the different types of sin the Chabad website has an article describing the three main types of sin -- only the less serious of which could be atoned for with sacrifice. Most required repentance, charity, and atonement to be forgiven for those sins. . . Link.
This sinning, all too human, subject of this psalm admits to sinning and to putting his trust in G-d. So should we all. . .