The hearing and believing of the the natural man, does not involve understanding.
Because they are not Born of God they are prevented from spiritually hearing God's Words.
John 8:43, 47
43 Why do ye not understand my speech? even because ye cannot hear my word.
47 He that is of God heareth God's words: ye therefore hear them not, because ye are not of God.
The word for "hearing" is akouō: to comprehend, to understand
And the word "understandeth" in Mat. 13:23 is the Gr. word syniemi and means:
i.e. to understand: the man of understanding
Many in false religion say they "believe" in God, or Christ, but because they are not Born of God, their own work of believing is merely a work of the flesh they do which cannot please God Rom. 8:8.
Mat. 13:20- 21
But he that received the seed into stony places, the same is he that heareth the word, and anon with joy receiveth it; 21 Yet hath he not root in himself, but dureth for a while: for when tribulation or persecution ariseth because of the word, by and by he is offended.
John 8:31
31 Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed;
But the ones who reject the Truth of the Gospel of the Grace of God are more than likely Vessels of dishonour fitted for destruction Rom. 9:23.
Rom. 2:5
But after thy hardness and impenitent heart treasurest up unto thyself wrath against the day of wrath and revelation of the righteous judgment of God;
The word "heart" here is the Gr. word "kardia" and one of the meanings is:
the centre and seat of spiritual life
the soul or mind, as it is the fountain and seat of the thoughts, passions, desires, appetites, affections, purposes, endeavours
of the understanding, the faculty and seat of the intelligence