Making a promise in itself is not sinful. But, if a promise is made and not fulfilled, then it is sin.
Matthew 5:37 teaches us to 'let our yes be 'yes', and our no be 'no'. If you make a promise, be sure to keep it. Be thoughtful when you make a promise. Make sure you have the power to keep it.
Carla quoted Matthew verse 37. In verses 33-36 Jesus said, "“Again you have heard that it was said to those of old, ‘You shall not swear falsely, but shall perform to the Lord what you have sworn.’ But I (Christ) say to you, Do not take an oath at all, either by heaven, for it is the throne of God, or by the earth, for it is His footstool, or by Jerusalem, for it is the city of the great King. And do not take an oath by your head, for you cannot make one hair white or black." I think if you use the words 'I promise' just off-handedly on something silly or just caught up in emotions & say it, that's not the same as taking a serious vow like it's speaking of here. If He considered it a sin every time I said 'I promised' before the dumbest comments, I'd stay in a state of sin.
Of course. Hope you're having a great day & am very blessed to have you on the site. I believe that if you make a promise just in passing for example: I may walk in my kid's room & say "I promise, I'm painting this room" but I never do, I don't believe that's a sin though I could be wrong. I don't believe it's held against me but maybe I shouldn't say that either. I believe Jesus was also clearing up the Mosaic Laws as well. For instance, He said You heard it said Thou shall not murder but I say.... He says the same thing here 'But I say...." Hope I made sense. If not, feel free to ask. Sometimes I go off on rabbit trails.