On Synergism (the good kind)
Another bit of Gerhard from the tomb of forgotten theology: we cooperate with God in our sanctification and He rewards our good works with eternal rewards. That's one reason why the rewards are promised - so that we will not lose heart in struggling against our sin and living righteously. This volume on Free Will and Free Choice is going to be worth every penny when CPH puts it out (and I don't get royalties, so no conflict of interest).
If rewards and punishments in the forum of divine judgment are under discussion, response must be made differently, for rewards are either of this life or of the life to come, as also are punishments. God rewards the external discipline even of the unregenerate person with the rewards of this life or temporal rewards and regularly punishes atrocious sins with atrocious punishments so that societies may be preserved. In Exod. 1:21, because “the Egyptian midwives feared God, He built homes for them.” God rewards only the works of the reborn with eternal rewards, and He does that not out of condign merit but out of gracious mercy, nor does God crown anything in us except His own gifts. In this way the question pertains to the reborn who, we do not deny, are coworkers [συνέργους] with God in good works, because the will, now freed from the yoke of sin, cooperates by virtue of new powers granted by the Holy Spirit.