In shaa’ Allaah I will make frequent and brief posts here: http://sunnianswers.tumblr.com/
Ibn Kathiir states: “Aļļaah made it clear that He does not have a child, unlike what the ignorant idolaters claimed with regards to the angels, and the obstinate among the Jews and the Christians claimed with regards to Al-^Uzayr and Jesus, and said (what might be literally translated as): “If Aļļaah had willed to take a child for Himself, then He would have selected from what He creates whatever He wills.” That is, it would have been different from what they claim. And this condition (of “If Aļļaah had willed”) is not necessary to happen, or even be possible (in the mind’s eye). Actually, it is completely impossible that it could be. It is only meant to show their ignorance in their claim.”
Then Ibn Kathiir gave a couple of similar uses of “if” for impossibilities, and said, “it is allowed to make a statement conditional upon something impossible for some purpose of the speaker.”[1]
Accordingly, the meaning is: ” if it was possible, and it is not, then it still would not be the angels, Jesus, or Al-^Uzayr.” In other words, the purpose is to show that their claim about them is wrong from several viewpoints, not only in the sense that it is impossible that Aļļaah should have a child.
[1] تفسير ابن كثير (7 / 85) : ثم بين تعالى أنه لا ولد له كما يزعمه جهلة المشركين في الملائكة، والمعاندون (6) من اليهود والنصارى في العزير، وعيسى فقال: { لَوْ أَرَادَ اللَّهُ أَنْ يَتَّخِذَ وَلَدًا لاصْطَفَى مِمَّا يَخْلُقُ مَا يَشَاءُ } أي: لكان الأمر على خلاف ما يزعمون (7) . وهذا شرط لا يلزم وقوعه ولا جوازه، بل هو محال، وإنما قصد تجهيلهم (8) فيما ادعوه وزعموه، كما قال: { لَوْ أَرَدْنَا أَنْ نَتَّخِذَ لَهْوًا لاتَّخَذْنَاهُ مِنْ لَدُنَّا إِنْ كُنَّا فَاعِلِينَ } [الأنبياء:17]{ قُلْ إِنْ كَانَ لِلرَّحْمَنِ وَلَدٌ فَأَنَا أَوَّلُ الْعَابِدِينَ } [الزخرف:81]، كل هذا من باب الشرط، ويجوز تعليق الشرط على المستحيل لقصد المتكلم. وقوله: { سُبْحَانَهُ هُوَ اللَّهُ الْوَاحِدُ الْقَهَّارُ } أي: تعالى وتنزه وتقدس عن أن يكون له ولد، فإنه الواحد الأحد، الفرد الصمد، الذي كل شيء عبد لديه، فقير إليه، وهو الغني عما سواه الذي قد قهر الأشياء فدانت له وذلت وخضعت.