1 Corinthians 15:14, 58

and if Christ has not been raised, then our preaching is vain [useless, amounting to nothing], and your faith is also vain [imaginary, unfounded, devoid of value and benefit—not based on truth].

Therefore, my beloved brothers and sisters, be steadfast, immovable, always excelling in the work of the Lord [always doing your best and doing more than is needed], being continually aware that your labor [even to the point of exhaustion] in the Lord is not futile nor wasted [it is never without purpose].