Adam Clarke on 2 Peter 2:21

For it had been better for them not to have known - For the reasons assigned above; because they have sinned against more mercy, are capable of more sin, and are liable to greater punishment.

The holy commandment - The whole religion of Christ is contained in this one commandment, "Thou shalt love the Lord thy God with all thy heart, with all thy soul, with all thy mind, and with all thy strength; and thy neighbor as thyself." He who obeys this great commandment, and this by the grace of Christ is possible to every man, is saved from sinning either against his God or against his neighbor. Nothing less than this does the religion of Christ require.