I would take it, on the same side as Strathos (more or less). I think Sin is not just an act, but an identity, and the identity of a person only changes through a lifetime, so the process of "becoming holy" takes the hole life. Also, holiness is a relative concept that depends on the subject.
Yes, if someone is talking about if one can be holy, one needs to be clear about what the person means by "holy".
God wants us to become "holy and without blame before Him in love." (see Ephesians 1:4)
So, the holiness which God desires has to do with how His love changes us to become "as He is" "in this world." (see 1 John 4:17)
So, it is not only about if we do certain wrong things or not. But also, I would say, holiness has to do with how love has us becoming in our feelings and emotions and attitudes and thinking and how we are "responding" . . . "because of how God's love is curing the character of our personality"
And to me Hebrews 12:4-11 means we all always will still need more correction of our character (Hebrews 12:8), and we need to seek our Father for this correction (Hebrews 12:10), and His correction will make us become "partakers of His holiness" (Hebrews 12:10) and have us enjoying "the peaceable fruit of righteousness". (Hebrews 12:11)
And Paul says, "Do all things without complaining and disputing, that you may become blameless and harmless, children of God without fault in the midst of a crooked and perverse generation" (in Philippians 2:14-16).
So, included in the process of becoming holy in love, I would say, is stopping all our "complaining and disputing" . . . which God does say to do, not in the next life, but right "in the midst of a crooked and perverse generation". So, if He commands this, I would say He considers this realistic; however, I do understand it takes the almighty power of God's love to cure our character so we succeed in this
So, if this is what you mean by becoming holy, I would say the Bible says yes this is realistic . . . possible with God
