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.
–Acts 4:31 (KJV)–

God gave them greater degrees of his Spirit, which was what they prayed for. Their prayer, without doubt, was accepted, for it was answered: They were all filled with the Holy Ghost, more than ever; by which they were not only encouraged, but enabled to speak the Word of God with boldness, and not to be afraid of the proud and haughty looks of men. The Holy Ghost taught them not only what to speak, but how to speak. Those that were endued habitually with the powers of the Holy Ghost had yet occasion for fresh supplies of the Spirit, according as the various occurrences of their service were. They were filled with the Holy Ghost at the bar (Acts 4:8), and now filled with the Holy Ghost in the pulpit, which teaches us to live in an actual dependence upon the grace of God, according as the duty of every day requires; we need to be anointed with fresh oil upon every fresh occasion. As in the providence of God, so in the grace of God, we not only in general live, and have our being, but move in every particular action, Acts 17:28. We have here an instance of the performance of that promise, that God will give the Holy Spirit to those that ask him (Luke 11:13), for it was in answer to prayer that they were filled with the Holy Ghost: and we have also an example of the improvement of that gift, which is required of all on whom it is bestowed; have it and use it, use it and have more of it. When they were filled with the Holy Ghost, they spoke the word with all boldness; for the ministration of the Spirit is given to every man, to profit withal. Talents must be traded with, not buried. When they find the Lord God help them by his Spirit, they know they shall not be confounded, Isaiah 50:7.

–MATTHEW HENRY COMMENTARY ON ACTS 4:31–

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Dr. John R. Rice, Why Preach Against Sin? (SLP, 1946), p. 4. “Other preachers, who do not have the courage or the conviction or the devotion to preach hard against sin, rationalize and excuse their failure. Sometimes they pretend that love and kindness of heart prevent their preaching against sin. They do not want to ‘hurt their testimony’ by offending anybody! How much nicer such preachers are than Jesus Christ Himself who preached so plainly against sin!”

Leonard Ravenhill, Why Revival Tarries (Bethany House, 1959), chs. 3, 7, 9, 13.

David Wilkerson, Hey, Preach…You’re Comin’ Through! (Revell, 1968).