And when they had prayed, the place was shaken where they were assembled together; and they were all filled with the Holy Ghost, and they spake the word of God with boldness. The place was shaken - This earthquake was an evidence of the presence of God, and a most direct answer to their prayer, as far as that prayer concerned themselves. The earthquake proclaimed the stretched-out arm of God, and showed them that resistance against his counsels and determinations must come to nought.
And they were all filled with the Holy Ghost - And, in consequence of this, they spake the word of God with boldness; a pointed answer to a second part of their request, Acts 4:29. A right prayer will always have a right and ready answer. Though these disciples had received the Holy Spirit on the day of pentecost, yet they were capable of larger communications; and what they had then received did not preclude the necessity of frequent supplies, on emergent occasions. Indeed, one communication of this Spirit always makes way and disposes for another. Neither apostle nor private Christian can subsist in the Divine life without frequent influences from on high. Had these disciples depended on their pentecostal grace, they might have sunk now under the terror and menaces of their combined and powerful foes. God gives grace for the time being, but no stock for futurity, because he will keep all his followers continually dependent on himself.
With boldness - Παντι τῳ θελοντι πιστευειν, To all who were willing to believe, is added by DE, two others, Augustin, Irenaeus, and Bede.