28 Therefore thus says the Lord, Behold, they that serve Me shall eat, but you shall be hungry; behold, they that serve Me shall drink, but you shall be thirsty. Note that one clause contains two different tenses, in order to teach truth concerning two different times; They that serve Me shall eat. Present piety is rewarded with a future prize, and similarly present godlessness shall suffer the penalty of future thirst and hunger. Then He adds, Behold, they that serve Me shall rejoice with gladness, but you shall cry for sorrow of your heart, and shall howl for vexation of spirit.
Here again, as before, there is a revelation for the future and for the present. They who serve now shall rejoice with gladness, while they who do not serve shall abide in crying and howling through sorrow of heart and vexation of spirit. He proceeds, For you shall leave your name for a rejoicing unto My chosen, but the Lord shall slay you. These words, dealing with a future time, are addressed to the carnal Israel, which is taunted with the prospect of having to surrender its name to the chosen of God. What is this name? Israel, of course; for to Israel the prophecy was addressed. And now I ask, What is Israel today? The Apostle gives the answer:— They who are in the spirit, not in the letter, they who walk in the Law of Christ, are the Israel of God.
Source: On the Trinity (New Advent)