قَالَ كَذَلِكِ قَالَ رَبُّكِ هُوَ عَلَيَّ هَيِّنٌ
He said;"Thus said your Lord;`That is easy for Me (Allah)..."
This means that the angel said to her in response to her question, "Verily, Allah has said that a boy will be born from you even though you do not have a husband and you have not committed any lewdness. Verily, He is Most Able to do whatever He wills."
Due to this, he (Jibril) conveyed Allah's Words,
وَلِنَجْعَلَهُ ايَةً لِلنَّاسِ
And (We wish) to appoint him as a sign to mankind,
This means a proof and a sign for mankind of the power of their Maker and Creator, Who diversified them in their creation.
He created their father, Adam, without a male (father) or female (mother). Then, He created Hawwa' (Adam's spouse) from a male (father) without a female (mother). Then, He created the rest of their progeny from male and female, except `Isa. He caused `Isa to be born from a female without a male.
Thus, Allah completed the four types of creation (of the human being), which proves the perfection of His power and the magnificence of His authority. There is no god worthy of worship except Him and there is no true Lord other than Him.
Concerning Allah's statement,
وَرَحْمَةً مِّنَّا
and a mercy from Us,
This means, "We will make this boy a mercy from Allah and a Prophet from among the Prophets. He will call to the worship of Allah and monotheistic belief in Him.
This is as Allah, the Exalted, said in another Ayah,
إِذْ قَالَتِ الْمَلَـيِكَةُ يمَرْيَمُ إِنَّ اللَّهَ يُبَشِّرُكِ بِكَلِمَةٍ مِّنْهُ اسْمُهُ الْمَسِيحُ عِيسَى ابْنُ مَرْيَمَ وَجِيهًا فِي الدُّنْيَا وَالاٌّخِرَةِ وَمِنَ الْمُقَرَّبِينَ
وَيُكَلِّمُ النَّاسَ فِى الْمَهْدِ وَكَهْلً وَمِنَ الصَّـلِحِينَ
(Remember) when the angels said;"O Maryam! Verily, Allah gives you the good news of a Word from Him, his name will be Al-Masih, `Isa, the son of Maryam, held in honor in this world and in the Hereafter, and will be one of those who are near to Allah. And he will speak to the people, in the cradle and in manhood, and he will be one of the righteous. (3;45-46)
This means that he will call to the worship of his Lord in his cradle and while and adult.
Concerning His statement,
وَكَانَ أَمْرًا مَّقْضِيًّا
and it is a matter (already) decreed (by Allah).
This is the completion of Jibril's dialogue with Maryam. He informed her that this matter was preordained by Allah's power and will.
Muhammad bin Ishaq said,
وَكَانَ أَمْرًا مَّقْضِيًّا
(and it is a matter (already) decreed (by Allah),
"This means that Allah determined to do this, so there is no avoiding it.