Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
In the sense of speaking evil of God this word is found in Ps. 74:18; Isa. 52:5; Rom. 2:24; Rev. 13:1, 6, 16:9, 11, 21. It denotes also any kind of calumny, or evil-speaking, or abuse (1 Ki. 21:10; Acts 13:45, 18:6, etc.). Our Lord was accused of blasphemy when he claimed to be the Son of God (Mt. 26:65; comp. Mt. 9:3; Mk. 2:7). They who deny his Messiahship blaspheme Jesus (Lk. 22:65; Jn. 10:36).
Blasphemy against the Holy Ghost (Mt. 12:31, 32; Mk. 3:28, 29; Lk. 12:10) is regarded by some as a continued and obstinate rejection of the gospel, and hence is an unpardonable sin, simply because as long as a sinner remains in unbelief he voluntarily excludes himself from pardon. Others regard the expression as designating the sin of attributing to the power of Satan those miracles which Christ performed, or generally those works which are the result of the Spirit's agency.