In Act 4, Scene 3, Malcolm and Macduff are in England discussing how bad things have gotten in Scotland. At this point, Malcolm is not sure who to trust. Malcolm wants to make sure that Macduff was not sent by Macbeth. Malcolm says, "This tyrant, whose sole name blisters out tongues, / Was once thought honest. You have loved him well; / He hath not touch'd you yet." (IV.iii.14-16). Macbeth has killed Malcolm's father (Duncan) but he hasn't harmed Macduff or his family, so Malcolm wants to be sure that Macduff is on his (Malcolm's) side. Macduff confirms that he is, saying, "I am not treacherous." (We learn later in this scene that Macduff's wife and child are killed by Macbeth's hired murderers.)
Malcolm reasons that if Macbeth was once loyal and is now a tyrant, it is difficult to know who to trust.
Angels are bright still, though the brightest fell.
Though all things foul would wear the brows of grace,
Yet grace must still look so. (IV.iii.25-27)
Even Lucifer ("brightest") was once an angel, but he (like Macbeth) fell from grace. Malcolm is still unsure if he can trust Macduff. Malcolm wants to make sure that Macduff is not like Macbeth, a once loyal angel who might fall from grace and betray him. Malcolm wants to make sure that Macduff is not a foul thing just wearing the "brows grace" as a disguise.