But be you strong, and don't let your hands be slack; for your work shall be rewarded. — 2 Chronicles 15:7
Let us not be weary in doing good, for we will reap in due season, if we don't give up. — Galatians 6:9

Author
Charles Spurgeon
Summary: Although God had done great things for Asa and Judah, they remained weak, wavering in their ways, and irresolute in heart. They needed to remember that the Lord would be with them as long as they remained with Him, but He would leave them if they forsook Him. The experience of the neighboring kingdom revealed both the consequences of rebellion and God’s mercy in response to repentance. This warning was meant to establish and strengthen them in righteousness, and it also calls us to persevere. God deserves all our energy: if His service is worth anything, it is worth everything. Our labor in the Lord is not in vain; we must devote ourselves to it diligently and wholeheartedly.
God had done great things for King Asa and for Judah, but they remained a weak nation. Their feet were very hesitant in the ways of the Lord, and their hearts very irresolute
, so they needed to be warned that the Lord would be with them while they were with Him, but that if they forsook Him, He would also leave them. They were also reminded of the sister kingdom, how badly it had fared in its rebellion, and how the Lord had been merciful to that kingdom when it showed repentance. The Lord’s purpose was to confirm them in their path and make them strong in righteousness. The same must be true of us. God deserves to be served with all the energy of which we are capable.
If the service of God is worth anything, then it is worth everything. We will find our best reward in the work of the Lord if we carry it out with resolute diligence. Our labor is not in vain in the Lord, and we know it. A half-hearted work will bring no reward; but when we give our whole soul to the cause, we will see prosperity
Source: http://www.spurgeon.com.mx/chequera/meditames/febrero.pdf
The Cheque Book of the Bank of Faith. Translated by Allan Román.