"Ye shall be witnesses unto me" Jesus

I read an article the other day that said most Christians today think it is optional to witness. Yet, Jesus didn't say it was an option when He gave the great commission Matt. 28:19. Also God gave a warning to us, that if we see someone doing sin that will lead him to hell and we do not warn him, his blood would be on our hands.

 Ezekiel 3:18-21 King James Version (KJV)
18 When I say unto the wicked, Thou shalt surely die; and thou givest him not warning, nor speakest to warn the wicked from his wicked way, to save his life; the same wicked man shall die in his iniquity; but his blood will I require at thine hand.

19 Yet if thou warn the wicked, and he turn not from his wickedness, nor from his wicked way, he shall die in his iniquity; but thou hast delivered thy soul.

20 Again, When a righteous man doth turn from his righteousness, and commit iniquity, and I lay a stumbling-block before him, he shall die: because thou hast not given him warning, he shall die in his sin, and his righteousness which he hath done shall not be remembered; but his blood will I require at thine hand.

21 Nevertheless if thou warn the righteous man, that the righteous sin not, and he doth not sin, he shall surely live, because he is warned; also thou hast delivered thy soul.