John Gill Commentary Luke 8:18

"Take heed therefore how ye hear: for whosoever hath, to him shall be given; and whosoever hath not, from him shall be taken away even that which he thinketh he hath." — Luke 8:18 (ASV)

Take heed therefore how ye hear
That you hear not in a careless and negligent manner, since what truths and doctrines you now hear with the ear, are to be preached by you unto others:

for whosoever has ;
that is, has knowledge of the doctrines of the Gospel, and has gifts and abilities to preach them to others:

to him shall be given ;
more knowledge, and by using his gifts they shall be increased:

but he that has not ;
true, solid, spiritual knowledge of divine things, though he has had considerable advantages and opportunities of learning it, as the apostles especially had:

from him shall be taken, even that which he seems to have ;
or "that which he thinks he has", as the Syriac version renders it; that which he seemed to others to have, or thought himself he had: the knowledge he had of truth, and which was rather a show of knowledge than real, shall be taken from him; his seeming gifts and parts shall die, and vanish away, and he shall be left to fall into ignorance, error, and heresy. Observe that this is to be understood not of internal grace, and experimental knowledge, but of speculative notions of the Gospel, and of external gifts; and so furnishes out no argument against the final perseverance of real saints; (See Gill on Matthew 13:12). (See Gill on Matthew 25:29).