He will reward works with what He gave freely before works

Calvin's Institutes (1559)
Book 3 of 4 - How We Receive the Grace of Christ

Chapter 18 - Works righteousness wrongly inferred from reward

1-4 - Passages about reward do not make works the cause of salvation

1. What does Scripture mean, to say we will be rewarded according to our works? Matt 16:27; 2 Cor 5:10; Rom 2:9-10, 6; John 5:29; Matt 25:34-35; Prov 12:14; Isa 3:11; Prov 13:13; Matt 5:12; Luke 6:23; 1 Cor 3:8. This is about sequence of events, not cause. Those justified are afterward glorified - Rom 8:30. Our work is not opposed to grace. God began the work, and will finish it - Phil 1:6.

2. Our rewards is an inheritance given by free adoption, not as wages.

Eph 1:5-6, 18; Gal 4:7, 30; Matt 25:34; Col 3:24. God made sovereign, free promises to Abraham, Gen 15:5; 17:1; 18:18, though it sounds like Abraham merited it in Gen 22:16-18. God has no reason to favor us, except by His mercy in Christ.

3. God spurs us on with promised rewards for works, which reward He promised us before in the Gospel - Col 1:4-5. The parable of the 11th hour vineyard workers shows that our work does not merit reward - the reward is based on grace - Matt 20:1ff. Eternal life is the fruition of our works, not theearned reward of them. Mark 10:30; Matt 25:34; Rom 8:18; 1 Pet 1:9.

4. God gives comfort in our earthly affiction and restrains our pride with promised reward - this is recompense for our trials, not earning your way to the reward - 1 Cor 15:19.