"That we may not lose," or be deprived of, "what we have worked for." This means that our work and our labor in ordering your eternal salvation should not be in vain. It also means that the fruit of your work, in receiving the teaching and ordering your life, should not perish.
"Someone of that kind may perhaps say: if I do not believe in the coming of Christ in the flesh, yet spend my life in good works, can I not stand alongside the pious through these works? Can I not receive a reward for them?
"The Apostle destroys such an objection beforehand. He says that one who denies the coming of Christ in the flesh should not even think of receiving the full recompense for works that awaits the truly faithful. Nor should he think of being numbered among the perfectly pious.
"For if he looks with contempt upon Him who came to teach men the perfect knowledge of God, how can he still be pious, when he despises the very Teacher of divine things? No, such a man is godless" (Theophylact).
"But that we may receive a full reward": that both we and you may receive a great reward in heaven (Mt 5:12). That is, we Apostles, the ministers of your eternal life, may by God's mercy receive wages for our labor and work (1 Cor 3:5-15). And you, the faithful, if you remain in faith and virtue despite every trial and temptation, will attain eternal life, which is the end of your faith and virtue (cf. Mt 10:42; 1 Cor 9:25; Heb 4:10; 1 Pet 1:9).
The Apostles, as ministers of men's salvation, undoubtedly received and will receive a reward for ordering men's eternal salvation, even for those who did not stand firm in faith and good life. But here the Apostle speaks of a full or perfect reward. According to the demand of apostolic love, such a reward is possible only when the joy of those who labor, or have labored, is full and perfect (2 Jn 1:12; 1 Jn 1:4; Jn 15:11; Jn 16:24).
According to the teaching of the holy Apostle Paul, those who order the salvation of men and those being so ordered are subject, in his figurative expression, to fire as a testing action. If anyone's work endures, he will receive a reward for that work. If anyone's work is burned, he will suffer loss. He himself will be saved, yet so as through fire. This points to the grievous moral state of the minister whose work is burned (1 Cor 3:12-15).
But when those who are taught remain in faith and piety, and those who teach see their progress, then the reward for both is full, perfect, and great.