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Here is a conundrum, If we take what Jesus said in the Bible I find there is a problem with this particular sin - Could it be that the early church added it to make a point about something else and unfortunately our generation ran with it.
Matthew 6:14 For if you forgive men when they sin against you, your heavenly Father will also forgive you. But if you do not forgive men their sins, your Father will not forgive your sins.
Matthew 26:28 This is my blood of the covenant, which is poured out for many for the forgiveness of sins.
Matthew 7:21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’
John 14:23 Jesus answered him, “If anyone loves me, he will keep my word, and my Father will love him, and we will come to him and make our home with him.
John 5:24 Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life.
John 11:25 Jesus told her, "I am the resurrection and the life. Anyone who believes in me will live, even after dying. Everyone who lives in me and believes in me will never ever die. Do you believe this, Martha?"
Matthew 12:31-32 31 And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. 32 Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
However If we look closely - Jesus said every kind of sin and slander can be forgiven, Then he said that one sin will not be forgiven - How can this be?! You cannot say every sin and then say - except one!
Further if we look at the scriptures above Jesus explicitly says that those who believe in him will inherit eternal life! How can this be - Surely he should have put a stipulation that Blasphemy can cause one to loose this eternal life! Considering this sin is the Greatest of All and the Only one that can catch us out it is very odd that this was not elabourated on more! Not even by his desciples!