Be you therefore perfect as also your heavenly Father is perfect. (Matt. 5:48)
In what does Christian perfection consist?
Christian perfection consists in loving God above all things and all things in God. Its essence is to love God above all things, for by so doing we shall certainly also love all other things in Him, namely, as He wills, as much as He wills, and because He wills.
Are we justified in thus representing that Christian perfection consists before everything else in the love of God?
We are justified, for "love is the fulfilling of the law"; in other words, whoever loves God above all things has fulfilled the whole law perfectly. "Above all things have charity, which is the bond of perfection"; this means that charity and perfection are interwoven in the closest possible manner; it is the teaching of St. Paul, who had himself attained perfection and knew well in what it consists.
St. Augustine, St. Gregory, St. Thomas all agree with the holy apostle on this point.
Moreover, it is easy to see why we must seek Christian perfection in the love of God before everything else. There are many other glorious and beautiful virtues, such as humility, purity of heart, mercy. But if anyone of them alone constitutes Christian perfection it must necessarily be that one which is pre-eminent among them, the most sublime, the queen of them all. Which would we describe in these terms? Surely the love of God. The theological virtues rank before the moral virtues, and of them charity is the first and the greatest. "Now there remain faith, hope, charity, these three; but the greatest of these is charity." All other virtues are but steps leading to the love of God.
If any single virtue comprises in itself the whole of Christian perfection, it must be one capable of including and implying all the others; and this is what the love of God does. If any man love God above all things, he will readily believe His word, hope firmly in His promises, and endeavor according to God's holy will to practice humility, brotherly love, mercy, and all the other virtues pleasing to Him.
Therefore, Christian perfection depends before everything else on love of God. It would be a mistake to think it consisted in long prayers, austerities, almsgiving, or the working of miracles. Perfection under some circumstances requires such things, or they may be marks and evidences of it, but they do not in themselves constitute it.
"If I should have all faith, so that I could remove mountains, and have not charity I am nothing." The marks of perfection are the same as those by which we recognize true love of God.
How can we tell whether we love God?
We love God if we do His holy will, that is, if we keep His commandments.
Popular Sermons on the Catechism, Bamberg & Thurston, 1914