In light of the resurrection
Therefore, my beloved brethren, be ye steadfast, unmoveable, always abounding in the work of the Lord, forasmuch as ye know that your labour is not in vain in the Lord. 1 Cor. 15:58
With these words, the apostle Paul ends his famous chapter on the critical importance of the doctrine of the resurrection of the dead. A summary of Paul's argument can be found in verse 22: "as in Adam all die, even so in Christ shall all be made alive." Those who are in Christ and who are now living in bodies characterized by corruption and weakness and dishonor will one day because of our Lord's victory over death be raised in bodies that are incorruptible, glorious, powerful - through the same Spirit who raised Christ from the dead (ver. 42-44; cf. Rom. 8:9-11).
However, this is not just a doctrine to be believed, though it is that. It is not just a way to characterize orthodoxy over against heterodox understandings of the work of Christ, though it is that. It is as well a truth that ought to make us "steadfast, unmoveable, always abounding in the work of the Lord." This is a truth that ought to cause those who are wavering in the face of opposition to persevere; it is a truth that ought to cause those who are ready to quit laboring to abound in the work of the Lord.
The reason for this is found in the last part of the verse: "forasmuch as ye know that your labor is not in vain in the Lord." If Christ is not risen, our faith is vain. But Christ is risen, the firstfruits of those who have already died and those who will die, and that means that nothing done for Christ can be in vain (1 Cor. 15:13-20). Hear this, weary Christian: the labor that you are giving and have given for the sake of God's kingdom has eternal value, and it has eternal value because of our Lord's redemptive work which will forever be exemplified in renewed and resurrected people.
Let us therefore live in light of the future resurrection of the dead. Let us labor in light of eternity, an eternity which we will enjoy with purified souls in renewed bodies forever serving the Lord with increasing gladness and joy.