I was revisiting this question the other day and came across an excellent answer from Ligonier’s I would like to share here.
“Van Lodenstein (who taught Semper Reformanda) was a Reformed pietist and part of the Dutch Second Reformation. As such, his religious concerns were very similar to those of the English Puritans.
They all believed that once the externals of religion had been carefully and faithfully reformed according to the Word of God, the great need was for ministers to lead people in the true religion of the heart.
They saw the great danger of their day not as false doctrine or superstition or idolatry, but as formalism. The danger of formalism is that a church member could subscribe to true doctrine, participate in true worship in a biblically regulated church, and yet still not have true faith.
As Jesus had warned against the Pharisees of His day, citing the prophet Isaiah, “This people honors me with their lips, but their heart is far from me” (Matt. 15:8).
The part of religion that always needs reforming is the human heart. It is vital religion and true faith that must be constantly cultivated. Formalism, indifferentism, and conformism must all be vigorously opposed by a faithful ministry.”