وكيف أخاف ما أشركتم
And how should I fear those whom you associate...,
means, how should I fear the idols that you worship instead of Allah.
وﻻ تخافون أنكم أشركتم بالله ما لم ينزل به عليكم سلطانا
while you fear not that you have joined in worship with Allah things for which He has not sent down to you any Sultan.
According to Ibn Abbas and others among the Salaf,
meaning, proof,
Allah said in similar Ayat;
أم لهم شركاء شرعوا لهم من الدين ما لم يٲذن به الله
Or have they partners who have instituted for them a religion which Allah has not allowed! (42:21)
and,
إن هى إﻻ أسماء سميتموها أنتم وءاباوكم ما أنزل الله بها من سلطن
They are but names which you have named, you and your fathers, for which Allah has sent down no authority. (53:21)
His statement,
فٲي الفريقين أحق باﻻمن إن كنتم تعلمون
(So) which of the two parties has more right to be in security! If you but know.
means, which of the two parties is on the truth, those who worship Him in Whose Hand is harm and benefit, or those who worship what cannot bring harm or benefit, without authority to justify worshipping them Who among these two parties has more right to be saved from Allah's torment on the Day of Resurrection.
Allah said,
الذين آمنوا ولم يلبسوا إيمانهم بظلم أوليك لهم اﻻمن وهم مهتدون