Truthfulness is the firmest road leading to God, and the truthful are fortunate travelers upon it. Truthfulness is the spirit and essence of action and the true standard of straightforwardness in thought; it distinguishes believers from hypocrites, and the people of Paradise from the people of Fire. Truthfulness is a Prophetic virtue in those who are not Prophets, by which "servants" share the same blessings as "kings." In the Qur'an, God Almighty described as being true both the one who communicated it and the one who confirmed it: He who came with the truth and he who declared that it is true (39:33).
Truthfulness can be defined as struggling to preserve one's integrity and to avoid hypocrisy and lying, even in strained circumstances when a lie will cause release. Junayd al-Baghdadi says: A loyal, truthful one changes states at least forty times a day (in order to preserve personal integrity), while a hypocrite remains the same for forty years without feeling any trouble or unease (over his or her deviation). 
The initial and lowest degree of truthfulness is sincerity and behaving the same whether in public or in private. This is followed by being true in all thoughts, feelings, actions, and intentions. The truthful are those valiant people whose feelings, thoughts, and actions do not contradict one another; the most truthful are those heroes who are absolutely true in all of their imaginations, intentions, feelings, thoughts, actions, and gestures.
Using all of one's faculties and capacities to achieve truthfulness in all actions, adherence to a lofty ideal or cause, and loyalty to or steadfastness in it is an attribute of Prophethood. In the verse: Make mention of Abraham in the Book: he was a most truthful one, a Prophet (19:41), the Qur'an refers to this highest degree of truthfulness. Truthfulness is the primary attribute of all Prophets, and the strongest moral force or means-producing activity for serving Islam and the Qur'an. It is also a believer's greatest credit and most valid document in the Hereafter. God draws our attention to this significant fact: This is a day in which their truthfulness profits the truthful (5:119).
Truthfulness serves the Prophets, the purified and perfected scholars and the saints brought near to God, as a heavenly mount that, as quick as lightning, can carry them to the highest heights; lying, however, pulls Satan and his followers down to the lowest depths. Thoughts "fly upward" on the wings of truthfulness and increase in value, actions grow and flourish on the ground of truthfulness, and truthful supplications and prayers reach the Throne of Mercy and are welcomed.
Truthfulness is as effective as the "elixir" of God's Greatest Name. When asked about the Greatest Name, Bayazid al-Bistami answered:
Show me the Least Name among God's Names so that I may show you the Greatest One. If there is something as effective (for the acceptability by God of prayers and actions) as the Greatest Name, it is truthfulness. Whichever Name is recited truthfully, It becomes the Greatest.
Truthfulness caused the light of repentance to shine on the forehead of Prophet Adam, upon him be peace. Truthfulness served as a vessel of salvation for Prophet Noah, upon him be peace, when the world was flooded. Truthfulness carried Prophet Abraham, upon him be peace, to safety and coolness from the fire into which he had been thrown. Truthfulness elevates ordinary people to extraordinary heights, and is a key that opens the door to realms and realities beyond visible existence. One borne aloft by truthfulness cannot be detained from journeying upward, and doors are not closed in the face of one who uses this key. How apt are the words of Rumi in this respect:
The truthfulness of a lover affects even the lifeless;
Why then should it be found strange that it affects man's heart?
The truthfulness of Moses affected his staff and the mountain;
Nay, it also affected that great, splendid sea.
As for the truthfulness of Muhammad, it affected
The beautiful face of the Moon and also the shining Sun.
In many verses, the Qur'an relates that being a true believer depends upon one's integrity and truthfulness in words, actions, feelings, and innermost senses. It also regards such a degree of integrity and truthfulness as the basis of happiness in both this world and the next. The following are a few examples:
- Say: My Lord, cause me to enter with truth, and cause me to go out with truth, and grant me a power from Your Presence to help me. (17:80)
- Grant me a good reputation, being remembered for truthfulness, in later generations. (26:84)
- Give to those who believe good tidings that they have a true, sure footing in the sight of their Lord. (10:2)
- The righteous are in the Gardens and rivers, on the seats of truthfulness in the Presence of a Mighty King. (54:54-5)
Entering with truth, going out with truth, being remembered for truthfulness, true and sure footing, and a seat of truthfulness are all stations of and provisions for a long road extending from this world to the Hereafter. Since what takes place in this world yields fruit in the Hereafter, the truthful always pursue what is true, observe truthfulness when starting a task or moving to another place in the way of God, act and live in accord with the requirements of truthfulness, and bequeath truthfulness to succeeding generations. Their objective is to deserve eternal happiness in the Hereafter.
To become truthful in intentions and aims, believers must consciously decide to become truthful in their thoughts, decisions, and acts. This is the first step. Those so resolved must persist in truthfulness, regardless of the consequences, and refrain from whatever might shake their resolution. The second step is to maintain this worldly life only to support the truth and gain God's approval and pleasure. Such people are always aware of the defects and faults of their selfhood, and do not surrender to the world's attractions and change their conditions for worldly reasons. The third step is to establish truthfulness so firmly in one's conscience that it governs every facet of life. This is identical with the station of being pleased, which is explained in the Tradition: One who is pleased with God as the Lord and Islam as the religion and Muhammad as the Prophet has tasted the delight of belief. 
The greatest truthfulness and loyalty is for people to be pleased with the Lordship of God regardless of the treatment He dispenses, to accept Islam as the Divine system governing their lives, and to submit willingly to the guidance or leadership of the best of creation, upon him be peace and blessings. The way to true humanity lies in undertaking this grave responsibility, which is very difficult to fulfill perfectly.
Let us conclude with a fine couplet:
What befits man is truthfulness even if under threat;
God Almighty is the helper of the truthful.
 Al-Qushayri, Al-Risala, 211.
 Muslim, "Iman," 56; Nasa'i, "Jihad," 18.