Isaiah begins with "the voice of one crying in the wilderness." The voice prophesied was that of John the Baptist, which Scripture confirms in Malachi 3:1; Matthew 3:3; Mark 1:2-3; Luke 3:4; and John 1:23. Who would John be speaking to, proclaiming his message of repentance? To all who would "hear" him! Those "who have ears to hear" (see Matthew 13:9, 43, etc.), which would be all those with whom God is working, His firstfruits!
What did that "voice" say? What did he call on his audience to do? "[P]repare the way of the LORD." The instruction becomes more specific: ". . . make straight in the desert a highway for our God. Every valley shall be exalted and every mountain and hill shall be made low, the crooked places shall be made straight and the rough places smooth." Filling up valleys and removing the tops of mountains seems like a lot of work for one man. This is where the firstfruits come in. Why are we to do this? So that "the glory of the LORD shall be revealed, and all flesh shall see it together."
Albert Barnes, in his commentary on Isaiah written in 1851, remarks on these verses:
The idea is taken from the practice of Eastern monarchs, who, whenever they entered on a journey or an expedition, especially through a barren and unfrequented or inhospitable country, sent harbingers [forerunners] or heralds before them to prepare the way. To do this, it was necessary for them to provide supplies, and make bridges, or find fording places over the streams; to level hills, and construct causeways over valleys, or fill them up; and to make a way through the forest which might lie in their intended line of march.
Those who went before, to mark and improve the route, were the forerunners. They were "the scouts, the pioneers, the ones sent before a king to prepare the way," as forerunner is defined. Recall Daniel Boone and his party of thirty expert woodsmen laying out a 200-mile-long route. Over time, as more people came over the trail, it was improved, widened, and smoothed. It all began, however, with one man. That man then led others, and it multiplied from there.
John the Baptist was one man "crying in the wilderness," yet he prepared the way for the Son of God. Each of us, in our daily lives, interacts with family, coworkers, neighbors, and others who may know little or nothing of God and His Word. Our words and deeds could well pave the way for any of them to answer God's call at another time. Each of us has opportunities to set an example that will affect their lives, hopefully in a positive way. In this way, each of us is a forerunner, marking and improving the trail through the conduct of our lives.