Is it a mortal sin to LIE TO GOD about offering something for someone who really needs it and quiting before I complete it?

Would the lie be mortal in this situation:
There was a great opportunity for my sister’s friend’s agnotic conversion; her parents are Methodists through so it would be difficulter to convert her. I offered not doing this thing for 5 minutes to God. I think I said it like a promise, with I will and stuff like nodding-like. As a minute went I decided this was too hard and he would have to make the minutes go faster. He did. It changed to the next minute really soon I believe, that interval of one minute changed into like 40 at least on my clock which was encouraging. But I quit in the middle of the time I said I would do it. I think it was in my ability to do it the whole 5 minutes but I picked not to.

Would it be a mortal sin to pretty much lie to God in this situation.

I sorta especially said it to God the Father because lying to the Holy Spirit/Peter/the-church-there happened once in the bible, the guy who didn’t want to give his material wealth & lied saying he gave all he was supposed to, and the guy died. [Acts Chapter 5:3-14] But maybe that was just Peter’s decision for it to happen. So it was especially not to the Holy Spirit.

In the first place you cannot make a promise to God that is binding under pain of sin if you break it (such as a religious vow) at all if you are a minor, and even as an adult, not without permission of your bishop. So there is no way “breaking a promise to God” is a sin if it is not a binding vow. In the second place, your story illustrates the foolishness of making promises you cannot keep anyhow. Much better to stick to traditional penitential times, places, and practices, and limit those to what you are able to do.

Simply sit down and pray now, without a promise for the future, for your friend’s intention, and when the chance comes up to make some sacrifice or penance for that intention, do it, and tell God then why you are doing it, with another prayer for your friend.

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