Those who are sowing in tears will sing when they reap. (Ps.126)
We know from the Gospels that the seed is the Word of God (Matt. 13). This is primarily sown by Jesus Christ, but he also send out laborer to share in this work of his. In this Psalms, the sowing is accompanied by tears--at least the sowing that will bring a songful harvest. What are these tears? For us who walk by faith, it could refer to our blindness in this present life. We do not yet see the things that we hold by faith, so that when we go out to sow, we have no clear vision of what will sprout, or if anything will at all. Perhaps also, it refers to the suffering that comes with preaching the Gospel, and this is more likely. Christ, the first-fruits of our salvation was reviled, betrayed and ultimately died for the sake of our salvation. So also, his ministers will not be treated any differently. A servant is not greater than his master, nor is a messenger greater than the one that sent him. (Jn. 13) We will do all the things he did and suffer all that he did, if we are truly his disciples. But then we will also enjoy the harvest with him.
They go out, they go out, full of tears,
carrying seed for the sowing.
They come back, they come back, full of song,
carrying their sheaves. (Ps. 126)
Then the very next Psalm considers the Lord's help, and again the reward:
If the Lord does not build the house, in vain do its builders labor. If the Lord does not watch over the city, in vain does the watchman keep vigil.
Certainly we are the builders of an edifice founded upon Christ (Matt. 7) and also responsible for watching over the city. This Psalm is a reminder that we are not the chief-architect nor will our eyes be sufficient for protecting the city--the Lord does this and will do this. He not a mere assistant of ours, rather he has graciously allowed us to take part in the work that he could do alone.
Truly sons are a gift from the Lord, a blessing the fruit of the womb. The sons of youth are like arrows in the hand of a warrior. O the happiness of the man who has filled his quiver with these arrows! He will have no cause for shame when he disputes with his foes in the gateways. (Ps. 127)
These verses have a literal meaning about the blessings of children, but for those who are consecrated and without children according to the flesh, these verses have a spiritual meaning as well. The first is that the "sons" refer to our good actions. Psalm 137 talks about he who takes the little ones and dashes them against the rock. These "little ones" of Babylon are a sign of the first movements toward sin, and these must be altogether destroyed. So the sons of youth are healthy actions which are at once arrows in the hand of warrior, for a combat against sin and the evil one. Not only does it say sons, but also the fruit of the womb, which is explain in our Lord's command to bear much fruit (John 15, Matt. 7), again referring to our good actions which manifest the sort of tree we are, planted by streams of running water (Ps. 1). Indeed, if one who is found with only good actions will have no cause for shame when he disputes with his foes in the gateways. Of course, the most important gateway leads into heaven. Before entering, we will be subject to a judgment where our foes, our sins and the demons who accuse us, want to prevent us from entering, but these sons will prevent their dispute from winning.
I also thought that perhaps the sons of youth can refer to those who we win over by our preaching and example, indeed our spiritual sons. These too will prove a benefit at the judgment. He who turns a sinner from the error of his way will save his soul from death and cover a multitude of sin. (James 5:20) Although good works are the basis of this judgment at the entryway (Matt. 25), the Psalmist teaches these things only after the statements above that the Lord is the builder and watcher, who pours gifts on his beloved while they slumber.