Oaths
v.33 Jesus makes an allusion to the OT verses Ex 20:7, Lv 19:12, Nm 30:2 and Dt 5:11. The law forbade using the Lord’s name lightly. Once his name was invoked in a vow, it was a debt that had to be paid.
v.34-36 Don’t swear at all, Jesus now tells us. He gives as examples of inappropriate oaths: oaths by heaven, earth, Jerusalem, and one’s head. In 23:16-22 he will add the temple, the gold of the temple, the altar, and the gift on the altar. Jesus has in mind the importance of telling the truth in all circumstances. Vows were necessary in certain circumstances because we live in a sinful society, but God calls us to a higher standard.
v.37 Let your yes be yes and your no, no. Anything beyond this comes from evil.

Read Matthew 5:33-37