Contradiction with Matthew 22:14
Unlike Mark 4:20 which implies that everyone hearing the Word can be fruitful, Matthew 22:14 suggests that only a few who are called are chosen.
Matthew 22:14: Then went the Pharisees, and took counsel how they might entangle him in [his] talk.
Contradiction with Romans 3:10-12
While Mark 4:20 speaks of people bearing good fruit, Romans 3:10-12 indicates that no one does good and all have turned away.
Romans 3:10-12: As it is written, There is none righteous, no, not one:
Contradiction with Jeremiah 17:9
Mark 4:20 describes fertile hearts producing grain, but Jeremiah 17:9 suggests the heart is deceitful and wicked, contradicting its ability to bear good fruit.
Jeremiah 17:9: The heart [is] deceitful above all [things], and desperately wicked: who can know it?
Contradiction with Hebrews 6:4-6
Whereas Mark 4:20 shows progression towards fruitfulness, Hebrews 6:4-6 warns that it’s impossible to bring back to repentance those who have fallen away after receiving the Word.
Hebrews 6:4-6: For [it is] impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
Contradiction with John 15:5
Mark 4:20 depicts individuals yielding fruit on their own, while John 15:5 states that apart from abiding in Christ, one can do nothing.
John 15:5: I am the vine, ye [are] the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing. [without me: or, severed from me]