The Prophet (peace be upon him) said: "Nothing will weigh more heavily in the believer's scale on the Day of Judgment than excellent character. Allah detests the vulgar and foul-mouthed." [at-Tirmidhi, 2002; authenticated by al-Albaani]
He also said: "The finest among you are those with the best character." [al-Bukhaari, 6035; Muslim, 2321]
Regarding faith: Good character strengthens one's faith, while poor character weakens it — though it does not nullify faith entirely. The Prophet (peace be upon him) stated: "The most complete believers in faith are those with the finest character." [Abu Dawood, 4682] This indicates that deficient manners represent a shortcoming in the obligatory aspects of faith.
Regarding good deeds: According to the understanding of Ahl as-Sunnah, righteous deeds are only completely invalidated by associating partners with Allah (shirk). Major sins diminish faith but do not erase one's record of good deeds.
Regarding dawah: Poor character inflicts serious damage on the effort of calling people to Allah. People naturally recoil from someone with bad manners, rejecting their counsel with the response: "He should fix himself first!" Conversely, a person of noble character attracts others, and people willingly accept their guidance.
Bad conduct from religious individuals gives others a negative impression of practicing Muslims as a whole. When dealing with non-Muslims, it drives them further from Islam rather than drawing them closer.
Every Muslim should be conscious that their behavior either attracts people to Allah's path or pushes them away. Developing good character is not merely commendable — for those engaged in dawah, it is essential.