Step back this morning and look at the whole passage as one piece. Jesus calls his disciples to a standard so high that it should crush us. Let your yes be yes. Let your no be no. The very existence of contracts, witnesses, and notaries is, as Dale put it on Sunday, a sermon on human brokenness. We have not kept our word with God or with one another, and we know it.
If the sermon ended there, it would simply be another impossible demand. But the Sermon on the Mount is not standing alone in the gospel. It is standing under a cross. The one who said let your yes be yes is the one whose yes was always yes. The one who said let your no be no is the one whose no to sin and to Satan and to the seduction of comfort never wavered. Hebrews 6 says that God, when he made his promise to Abraham, swore by himself because there was no one greater. The writer of Hebrews calls this promise a sure and steadfast anchor of the soul.
Then Paul tells us in 2 Corinthians 1:20 that all the promises of God find their Yes in Jesus. He is the kept word of God to you. He kept every oath the Father ever made. He clothes you in a righteousness you could never generate, and by his Spirit he begins the slow work of remaking you into a person whose yes actually means yes. The standard does not crush the believer; it drives the believer to the cross, and from the cross to a quiet, lifelong reformation of the tongue.
If you belong to Christ this morning, hear the gospel. Your broken words are met by his perfect word. The Father’s verdict over you is yes, in Christ, forever. And by the Spirit who dwells in you, that yes is being formed into your speech, day by ordinary day, until the people in your life can plan on your word the way you plan on his.