Answer
Asalamu Alaikum Khurram,
Thank you very much for contacting About Islam with your question.
The answer is that it is absolutely, decisively, and indispensably important to keep a promise. Why is that so?
Islam teaches that the real rank of humans is in fact their eternal rank, which is their rank with God, and their rank in the Hereafter.
Now, let’s see what type of humans is at the lowest rank. The words of God, Mighty and Majestic, explain to our ever inquiring minds:
{The hypocrites will be in the lowest depths of the Fire: no helper will you find for them.} (An-Nisaa’ 4:145)
The reasons the hypocrites are in the deepest reach of the Hellfire is a spectacular manifestation of divine justice and of how deeply sick such individuals are.
Their sickness surely is such that it extends itself to forms of sick words and actions.
Now, what are the defining characteristics of such a sickness?
For that we will turn to the best human character, the one from whom all things meritorious emanate. The Prophet (peace be upon him-PBUH), the one who was sent to all to set a guiding example and to exemplify good character said:
There are three signs of a hypocrite: whenever he speaks, he lies; whenever he makes a promise, he breaks it; and whenever he is trusted, he betrays his trust. (Al-Bukhari)
It is added in another version:
Even if he fasts and prays and claims that he is a Muslim. (Muslim)
Islam is a religion centered on singling God our for all acts of devotion. At the heart of Islam is the Quran, the ultimate guide, which enlightens our sense with:
{O you who believe! Why do you say what you do not do? It is deeply abhorrent to Allah that you say what you do not do.} (As-Saff 61:2-3).
The point is, no doubt, clear.
To gather every verse, hadith, early report, story, admonition of a scholar, or aphorism of a sage on fulfilling a promise is surely beneficial in driving the point home.
However, the space here is enough to mention only a glimpse of that. And for that, the emphasis will be not on how virtuous such a value is, but rather, it will be on how such a value is among the main reasons one will reach, in the ranks of eternal existence in the Hereafter, the highest levels of existence.
There is not a single more telling proof of the importance of keeping a vow or a promise than that, in the word of God, Mighty and Majestic, a vow of a believer is by extension a vow from God:
{Fulfill the Covenant of God when you have entered into it, and break not your oaths after you have confirmed them; indeed you have made God your surety; for God knows all that you do.} (An-Nahl 16:91)
There are a few sections in the Quran that portray the moral makeup of the faithful and many of them put fulfilling promises in the center of the meritorious traits of the faithful.
Upon demonstrating one such a portrayal, I would like to make a final point. Keeping one’s promise is an expression of truthfulness, which is itself a prerequisite for piety, a trait which God loves. And what a great rank that is to be loved by God.
Here is a great outline of the reality of faith:
{It is not righteousness that you turn your faces towards East or West; but it is righteousness to believe in God and the Last Day, and the Angels, and the Book, and the Messengers; to spend of your substance, out of love for Him, for your kin, for orphans, for the needy, for the wayfarer, for those who ask, and for the ransom of slaves; to be steadfast in prayer, and practice regular charity; to fulfill the contracts which you have made; and to be firm and patient, in pain (or suffering) and adversity, and throughout all periods of panic. Such are the people of truth, the God-fearing.} (Al-Baqarah 2:177)
And here is the ultimate answer to your question:
{Those that keep their plighted faith and act aright,-verily God loves those who act aright.} (Aal `Imran 3:76)
Yes, keeping a promise, even if it seems simple, as the verse shows, is an adequate reason to gain the most precious of all things; the love of God Almighty.
Nothing, after all, is more important than the love of the Divine.
I hope this answers your question. Please keep in touch.
Walaikum Asalam.
Please continue feeding your curiosity, and find more info in the following links:
What Are The True Teachings of Islam?
Is Their Criteria for True Faith?