"That, trusted home,/ Might yet enkindle you unto the crown,/ Besides the thane of Cawdor. But 'tis strange./ And oftentimes, to win us to our harm,/ The instruments of darkness tell us truths,/ Win us with honest trifles, to betray's/ In deepest consequence.
The use of the phrase "The instruments of darkness tell us truths," could perhaps refer to the Elizabethan belief of the music of the spheres. Perhaps the 'instruments of darkness" relates to Banquo's recognition of how the Weird Sister's use of the supernatural will cause corruption to the universe as the music of spheres, the Elizabethan audience believed, was a reflection of a perfect God. Banquo therefore inadvertently warns Macbeth that by following the witches, he is indirectly denying God's perfection as he is seeking other means of a Higher Power and failing to recognise that God is more powerful.
Furthermore, the music of spheres expressed divine harmony which further justifies the notion that Macbeth's following of the Weird Sisters will lead to the corruption of nature- a reoccurring motif throughout the play.