"I say, therefore, that they sin against the Holy Spirit who, with evil intention, resist God's truth, although by its brightness they are so touched that they cannot claim ignorance. Such resistance alone constitutes this sin. For Christ, to explain what he had said, immediately adds: 'He who speaks against the Son of man will have his sin forgiven; but he who blasphemes against the Spirit will not be forgiven' [Matt. 12:32, 31 Vg.; cf. Luke 12:10; Mark 3:29]."
And:
"But they whose consciences, though convicted that what they repudiate and impugn is the Word of God, yet cease not to impugn it - these are said to blaspheme against the Spirit, since they strive against the illumination that is the work of the Holy Spirit. Such were certain of the Jews, who, even though they could not withstand the Spirit speaking through Stephen, yet strove to resist [Acts 6:10]." -- Institues III:III:22, Battles translation.
"But was there ever in the world a more deplorable proof of the littleness of human understanding, even in those that have honest hearts, and are desirous of knowing the truth! How is it possible, that any one who reads his Bible, can one hour remain in doubt concerning it, when our Lord himself, in the very passage cited above, has so clearly told us what that blasphemy is? 'He that blasphemeth against the Holy Ghost hath never forgiveness: Because they said, He hath an unclean spirit.' (Mark iii.29,30.) This then, and this alone, (if we allow our Lord to understand his own meaning,) is the blasphemy against the Holy Ghost: The saying, He had an unclean spirit; the affirming that Christ wrought his miracles by the power of an evil spirit; or, more particularly, that 'he cast out devils by Beelzebub, the prince of devils.' Now, have you been guilty of this? have you affirmed, that he cast out devils by the prince of devils? No more than you have cut your neighbour's throat, and set his house on fire. How marvellously then have you been afraid, where no fear is! Dismiss that vain terror; let your fear be more rational for the time to come. Be afraid of giving way to pride; be afraid of yielding to anger; be afraid of loving the world, or the things of the world; be afraid of foolish and hurtful desires; but never more be afraid of committing the blasphemy against the Holy Ghost! You are in no more danger of doing this, than of pulling the sun out of the firmament." -- from his Sermon LXXXVI, "A Call to Backsliders," The Works of John Wesley, Volume 6, pp. 524-525.
"A form of unforgiven sin is seen in the case of the unpardonable sin, which was committed only when Christ was here on earth, and which sin is not now possible, both because of the fact that Christ is not here as He was then nor is He in the same relation to the Holy Spirit, and because such a penalty as is imposed on those who committed the unpardonable sin sets up a direct contradiction of divine grace in salvation. There cannot be an unpardonable sin and a whosoever-will gospel at the same time." -- Systematic Theology,, Volume II, page 269.