Thank you for the theological question, “Does the gospel demand grace for the heretic as well as the sinner?”
The central word in this question is demand. The word, demand, is defined as an insistent and peremptory request, made as if by right.
As I view this question for thoughts is that the Gospel demands grace is a word choice that one must guard against two extremes. One pitfall is that the question implies that God is obligated to offer salvation to everyone and the other pitfall is that one doesn’t need grace for salvation.
Ephesians 2:8–9 avoids both extremes. The text says this, “8 For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— 9 not by works, so that no one can boast.” (NIV).
Here are some thoughts for one’s consideration:
A. The Gospel (message of salvation) is a gift from God (Eph. 2:8–9).
The giver of a gift is under no obligation to give the gift to anyone. S/he may hold or withhold the gift since the person who wants it can’t demand that giver give it to him or her. God is under no obligation to anyone to give the Gospel of salvation to anyone regardless of their moral condition.
To imply that God is obligated would mean that God has to ensure that everyone person hears and or believes the Gospel. To do otherwise would violate offering grace to everyone.
When we see a homeless or needy person, one is not obligated to give whatever the needy person demands from him. If it is an obligation, then it is not grace, but law.
B. The Gospel exclude the possibility of earning salvation by one’s own work.
The question may be interpreted that if grace is not demanded, then one can earn his/her own salvation. In other words it opens the possibility that one can by his own moral righteousness be awarded salvation.
That option of earning one’s own salvation is false. Paul states that no one is saved by his/her own good works. No human being regardless of their moral condition can redeemed himself by his/her goodness. The “good” person or the heretic are equally bankrupt before God.
What is demanded is faith. It is believing or trusting God’s promise of forgiveness of sins in one’s life because Jesus Christ paid for one’s sin. God’s offer of salvation is true based on his character of truthfulness and holiness. God can’t lie. The offer is real so the question then becomes will the “good” person or the heretic believe God’s promise of salvation apart from personal good works and received as a gift from God to him/her?
SUMMARY: The Gospel offers grace to the one who would believe it.
For more perspectives:
https://www.quora.com/Does-the-gospel-demand-grace-for-the-heretic-as-well-as-the-sinner