Deuteronomy 15:19 All the firstling males that come of thy herd and of thy flock thou shalt sanctify unto the Lord thy God: thou shalt do no work with the firstling of thy bullock, nor shear the firstling of thy sheep.
Sanctify means to make holy, or more literally, set apart for the Lord. We respond to Grace and Salvation by setting apart for the Lord a portion of our time and of our material goods. The principle here is that it will be the first and the best (without blemish) of our time and money. This response is a sign or token that our whole self and all that we have is the Lord’s. He saved us so that we belong entirely to Him.
In a non-farming society a practical response is to list the tithe as the first item of the monthly budget, and to not give the Lord what is left over after we have spent it on ourselves. And we give God the best of what we have (like a lamb without spot or blemish), not something we don’t need for self anyway. When the first and the best is set apart for the Lord it symbolizes that all of our life, time, work, play, energy, talents, and possessions is sanctified (set apart) by the Lord.
Jesus sanctified us forever and blessed us fully. Now we sanctify Him in our hearts. 1 Peter 3:15: “But sanctify (set apart) the Lord God in your hearts.” We signify that we have done so by giving Him the first and the best. “Seek first the kingdom of God.” We are all God’s and God is all.