Some argue that the Bible is contradictory concerning whether there is an unforgivable sin. The argument is that Matthew 12:31-32 and Mark 3:29 say that blasphemy of the Holy Spirit is an unforgivable sin, but passages such as Acts 13:39, Titus 2:13-14, and 1 John 1:9 say that no sin is unforgivable. However, there is a reasonable explanation that solves this alleged contradiction in the Bible.
31 Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. 32 And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
Matthew 12:31-32
but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin”—
Mark 3:29
and by him everyone who believes is freed from everything from which you could not be freed by the law of Moses.
Acts 13:39
13 waiting for our blessed hope, the appearing of the glory of our great God and Savior Jesus Christ, 14 who gave himself for us to redeem us from all lawlessness and to purify for himself a people for his own possession who are zealous for good works.
Titus 2:13-14
If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.
1 John 1:9
The most likely explanation for this supposed contradiction in the Bible is that the “unforgivable” sin in Matthew 12:31-32 and Mark 3:29 is unbelief. “Speaking against” the Son of Man or against the Holy Spirit obviously cannot mean simply saying words about them because there are many people in the Bible who have spoken words against Jesus and the Holy Spirit who have ended up having faith and being saved.
Blasphemy of the Holy Spirit, or the “unforgivable” sin, most likely refers to a hardness of heart against Jesus and the Holy Spirit that refuses to acknowledge them as God and Lord. Someone who has this hardness of heart will persist in unbelief, and will ultimately not be forgiven of sin.
There are some foundational principles that apply to all alleged and apparent contradictions in the Bible. To read more, see Bible Difficulties: Foundational Principles.
To read more answers to alleged and apparent contradictions in the Bible, see “Contradictions” in the Bible Answered.
These books are also excellent resources:
Evidence That Demands a Verdict: Life-Changing Truth for a Skeptical World - Josh and Sean McDowell
Little Pilgrim's Progress (Illustrated Edition) - Helen L. Taylor