Al-Isra: 110
"Say: Call upon Allah or call upon the Most Merciful..."
It is narrated from Ibn Abbas (may Allah be pleased with them both) that Abu Jahl heard him [the Prophet] saying: "O Allah, O Most Merciful." He said: "He forbids us from worshipping two gods, yet he calls upon another god." It is also said that the People of the Book said: "You rarely mention 'the Most Merciful,' yet Allah has mentioned this name frequently in the Torah," so this verse was revealed.
"Call" (ad‘u) here means "to name" (tasmiya), not "to summon" (nida’). It is a verb that takes two objects; you say: "I called him Zayd" (da‘awtuhu Zaydan). Then, one of the objects may be omitted when it is understood, so one says: "I called Zayd."
"Allah" and "the Most Merciful" (al-Rahman): The intent here is the names themselves, not the Named. The "or" (aw) is for choice. Thus, the meaning of "Call upon Allah or call upon the Most Merciful" is: "Name Him by this name or by that, and mention either this or that."
The tanwin in "anyyan" (ayyan) is a substitute for the genitive construction (mudaf ilayh). The "ma" is a particle of emphasis for the ambiguity inherent in "any" (ay), meaning: "Whichever of these two names you use to call or mention, 'to Him belong the Most Beautiful Names'."
The pronoun in "to Him" (lahu) does not refer to one of the two mentioned names, but rather to the Named—His Essence, Exalted is He—because naming applies to the Essence, not the name itself. The meaning is: "Whichever of them you call upon, it is good." He replaced that with the phrase "to Him belong the Most Beautiful Names" because if all His names are beautiful, then these two names are also beautiful, as they are among them. The meaning of them being the "most beautiful names" is that they are sufficient in their meanings of praise, sanctification, and glorification.
"And do not recite your prayer too loudly, nor too quietly..."
"Your prayer" (bi-salatika) is a case of an omitted genitive noun (mudaf), meaning "your recitation in your prayer," because it is not ambiguous. This is because "loudness" and "quietness" are qualities that alternate only regarding the voice, while prayer consists of actions and invocations. The Messenger of Allah (peace be upon him) used to raise his voice in his recitation, and when the polytheists heard it, they would mock and insult it. Thus, he was commanded to lower his voice. The meaning is: Do not recite so loudly that the polytheists hear you, and do not recite so quietly that those behind you cannot hear you; rather, "seek a way between" loudness and quietness—a middle path.
It is narrated that Abu Bakr (may Allah be pleased with him) used to lower his voice in his prayer, saying: "I am whispering to my Lord, for He knows my need." Umar (may Allah be pleased with him) used to raise his voice, saying: "I am driving away the devil and waking the sleepy." Thus, Abu Bakr was commanded to raise his voice slightly, and Umar was commanded to lower his slightly.
It is also said that the meaning is: Do not recite your entire prayer loudly, nor your entire prayer quietly, but seek a way between that by reciting loudly in the night prayers and quietly in the day prayers. Others say "your prayer" means "your supplication." Some hold that this verse was abrogated by His saying: "Call upon your Lord in humility and privately" (Al-A'raf: 55).
"Seeking a way" is a metaphor for adopting a middle course in recitation.
"And say: 'Praise to Allah, who has not taken a son, has had no partner in [His] dominion, and has no protector out of weakness,' and glorify Him with [great] glorification."
"Protector out of weakness" (wali min al-dhull): A helper against humiliation or one who prevents it because of His might. Or, it means He does not take anyone as a protector due to any humiliation [on His part] that would require a protector to remove it.