A.T. Robertson Commentary 1 Corinthians 15:58

"Wherefore, my beloved brethren, be ye stedfast, unmoveable, always abounding in the work of the Lord, forasmuch as ye know that your labor is not vain in the Lord." — 1 Corinthians 15:58 (ASV)

Be ye steadfast, unmovable (εδραιο γινεσθε, αμετακινητο). "Keep on becoming steadfast, unshaken." Let the sceptics howl and rage. Paul has given rational grounds for faith and hope in Christ the Risen Lord and Saviour. Note practical turn to this great doctrinal argument.

Work (εργον),

labour (κοπος, toil). The best answer to doubt is work.