After Allah affirms in this noble Surah that `Isa was a worshipper and servant of Allah and He mentioned his birth from Maryam without a father, He then begins refuting those who claim that He has a son. Holy is He and far Exalted is He above such description. Allah says,
(And they say: "The Most Gracious has begotten a son." Indeed you have brought forth) This means, "In this statement of yours."
شَيْئاً إِدّاً
(a thing Idda.) Ibn `Abbas, Mujahid, Qatadah and Malik all said, "Terrible." It has been said that it is pronounced Iddan, Addan, and Addan with elongation on the first vowel. All three of these pronunciations are known, but the most popular is the first. Allah said;
"Except the one who has taken a covenant with the Rahman (All-Merciful) " - 19:87.
Sayyidna Ibn ` Abbas ؓ has said that عَھد means a declaration of firm belief in the Unity of Allah Ta’ ala, while others say that عِھد means committing the Qur’ an to memory. In brief only those people will have the permission to intercede with Allah who stand firm by their covenant to adhere to the true faith. (Ruh)
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 87–91
When Truth in its authentic form appears before a man and still he ignores it, this disregard of his paves the way for Satan’s entry into his very soul. As a result, his mind is given over to obduracy. Now he is not able to accept any argument in a right and positive manner. God’s signs may appear in front of him, but he offers self-devised explanations for them and even uses them to bolster his rebellious attitude and his arrogance. One who assumes false supports to be his real supports, which he can safely lean on, falls a prey to the same foolishness time and again. But those who fear God consider God alone to be their real support. The fear of God makes the believers give no importance to such people as mislead others into the belief that they are their true support. Thus it is the believers alone who will be God’s guests of honour in the life hereafter.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.