Hebrews 5:14 But strong meat belongs to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil. 

‘Full age’ [Greek: teleios] is one who has reached full maturity as to age or qualification, or by initiation (1Cor. 14:20). Two characteristics of adults: Capable of instruction. Have powers of discrimination – know what is right and what is evil as stated in Scripture, not trying to bypass God’s Word by asking others whether something is right or wrong.

We are commanded to be perfect throughout Scripture and are disobedient if we fail to attain this state of being. Perfect in Greek is teleios which means to be mature and complete. We must be of full growth, as men, and not children, in Christian knowledge – thoroughly instructed and deeply experienced, as in 2Corinthians 2:6; 14:20; Ephesians 4:13; Philippians 3:15; Colossians 1:28; James 1:4; 3:2; Hebrews 5:14 and 1John 4:18.

‘Senses’ [Greek: aistheterion] organs of sense, such as the eyes, ears, nose, tongue, palate, fingers, and the nervous system in general. These organs properly exercised and employed give one immediate consciousness of sight, hearing, taste, smelling, and all feelings. There is something in the soul that corresponds with all these senses and through them, the soul is blessed and perfected.

‘Exercised’ [Greek: gumnazo] to train stripped of clothes, as in gymnastic exercises (5:14; 12:11; 1Tim. 4:7; 2Pet. 2:14).

‘Discern’ [Greek: diakrisis] to discriminate; the faculty of distinguishing; judgment (5:14; Rom. 14:1; 2Cor. 12:10).

There is no excuse for any Christian to not know what qualifies as sins to the death – those that cut us off immediately from God (Mar. 7:21-23; Rom. 1:28-32; 1Cor. 6:9-11; Gal. 5:19-21; Efe. 4:17-31; 5:3-7, 11-12, 18; Col. 3:5-10).