Objection 1. It would seem that in this life it is possible to fulfil this precept of the love of God. For according to Jerome [Pelagius, Exposit. Cath. Fid.] "accursed is he who says that God has commanded anything impossible." But God gave this commandment, as is clear from Deuteronomy 6:5. Therefore it is possible to fulfil this precept in this life.
Objection 2. Further, whoever does not fulfil a precept sins mortally, since according to Ambrose (De Parad. viii) sin is nothing else than "a transgression of the Divine Law, and disobedience of the heavenly commandments." If therefore this precept cannot be fulfilled by wayfarers, it follows that in this life no man can be without mortal sin, and this is against the saying of the Apostle (1 Corinthians 1:8): "(Who also) will confirm you unto the end without crime," and (1 Timothy 3:10): "Let them minister, having no crime."
Objection 3. Further, precepts are given in order to direct man in the way of salvation, according to Psalm 18:9: "The commandment of the Lord is lightsome, enlightening the eyes." Now it is useless to direct anyone to what is impossible. Therefore it is not impossible to fulfill this precept in this life.
On the contrary, Augustine says (De Perfect. Justit. viii): "In the fulness of heavenly charity this precept will be fulfilled: Thou shalt love the Lord thy God," etc. For as long as any carnal concupiscence remains, that can be restrained by continence, man cannot love God with all his heart.
I answer that, A precept can be fulfilled in two ways; perfectly, and imperfectly. A precept is fulfilled perfectly, when the end intended by the author of the precept is reached; yet it is fulfilled, imperfectly however, when although the end intended by its author is not reached, nevertheless the order to that end is not departed from. Thus if the commander of an army order his soldiers to fight, his command will be perfectly obeyed by those who fight and conquer the foe, which is the commander's intention; yet it is fulfilled, albeit imperfectly, by those who fight without gaining the victory, provided they do nothing contrary to military discipline. Now God intends by this precept that man should be entirely united to Him, and this will be realized in heaven, when God will be "all in all," according to 1 Corinthians 15:28. Hence this precept will be observed fully and perfectly in heaven; yet it is fulfilled, though imperfectly, on the way. Nevertheless on the way one man will fulfil it more perfectly than another, and so much the more, as he approaches by some kind of likeness to the perfection of heaven.
Reply to Objection 1. This argument proves that the precept can be fulfilled after a fashion on the way, but not perfectly.
Reply to Objection 2. Even as the soldier who fights legitimately without conquering is not blamed nor deserves to be punished for this, so too he that does not fulfil this precept on the way, but does nothing against the love of God, does not sin mortally.
Reply to Objection 3. As Augustine says (De Perfect. Justit. viii), "why should not this perfection be prescribed to man, although no man attains it in this life? For one cannot run straight unless one knows whither to run. And how would one know this if no precept pointed it out."
Source: Summa Theologica (New Advent)