He is attempting to push David and others salvation up to the day of Pentecost, denying it prior.
But he doesn't know what in the world he is talking about. The murderous 1 John 3:15 attitude still exists.
David and the others were clearly saved, yet commited egregious sins. He wants to damn them to hell as murderers, while muredering them himself. And, um, Calvin, and therefore us. That is exactly what is intended in all of that.
Yet they were all in Paradise, secure, saved, knew and experienced salvation until Christ died in time. Those who murder brothers in their hearts show they don't possess eternal life. Cain murdered Abel because he was righteous (because God saved him).