And men shall speak of the might of thy terrible acts - The force, the power of those things done by thee which are suited to inspire fear or reverence. The great power displayed in those acts shall be a ground or reason for celebrating thy praise. The manifestations of that power will so deeply impress the minds of people, that they will be led to speak of them.
And I will declare thy greatness - Hebrew, “And thy greatness, I will declare it.” In respect to that, I will recount it, or I will make it known to others.