When people say that it is impossible to attain perfection, to be once and for all free from the passions, or to participate fully in the Holy Spirit, we should cite Holy Scripture against them…for the Lord said: ‘Become perfect, as your heavenly Father is perfect’ (Matthew 5:48), perfection denoting total purity…and St Paul is saying the same as Christ when he writes: ‘…so that we may present every man perfect in Christ’ (Colossians 1:28); and: ‘…until we all attain to the unity of the faith and of the knowledge of the Son of God, to a perfect man, to the measure of the stature of the fullness of Christ’ (Ephesians 4:13).
Those who deny the possibility of perfection inflict the greatest damage on the soul in three ways. First, they manifestly disbelieve the inspired Scriptures. Then, because they do not make the greatest and fullest goal of Christianity their own, and so do not aspire to attain it, they can have no longing and diligence, no hunger and thirst for righteousness (Matthew 5:6); on the contrary, content with outward show and behavior and with minor accomplishments of this kind, they abandon that blessed expectation together with the pursuit of perfection and of the total purification of the passions. Third, thinking they have reached the goal when they have acquired a few virtues, and not pressing on to the true goal, not only are they incapable of having any humility, poverty and contrition of heart but, justifying themselves on the grounds that they have already arrived, they make no efforts to progress and grow day by day.
~St Makarios of Egypt (paraphrased by St Symeon Metaphrastis)