The Quran

وَقالَ الَّذينَ كَفَروا لَولا نُزِّلَ عَلَيهِ القُرآنُ جُملَةً واحِدَةً ۚ كَذٰلِكَ لِنُثَبِّتَ بِهِ فُؤادَكَ ۖ وَرَتَّلناهُ تَرتيلًا

Muhammad Asad

Now they who are bent on denying the truth are wont to ask, "Why has not the Qur'an been bestowed on him from on high in one single revelation?"¹ [It has been revealed] in this manner so that We might strengthen thy heart thereby - for We have soarranged its component parts that they form one consistent whole.² -

Lit., "in one piece" or "as one statement" (jumlatan wahidatan) - implying, in the view of the opponents of Islam, that the gradual, step-by-step revelation of the Qur'an points to its having been "composed" by Muhammad to suit his changing personal and political requirements.
I.e., free of all inner contradictions (cf. 4:82). See also 39:23, where the Qur'an is spoken of as "fully consistent within itself". The concise phrase rattalnahu tartilan comprises the parallel concepts of "putting the component parts [of a thing] together and arranging them well" as well as "endowing it with inner consistency". Inasmuch as full consistency and freedom from contradictions in a message spread over twenty-three years of a life as full of movement and drama as that of the Prophet does give a clear indication of its God-inspired quality, it is bound to strengthen the faith of every thinking believer: and herein lies, according to the Qur'an itself, the deepest reason. for its slow, gradual revelation. (When applied to the reciting of the Qur'an - as in 73:4 - the term tartil refers to the measured diction and the thoughtful manner in which it ought to be enunciated.)