First, we shall consider the Scriptures, and see what they tell us concerning this doctrine. This is not an attempt to make a private interpretation of the Scriptures. The Church is the ultimate authority of the manner in which Scripture must be interpreted, and thus we cannot make the final determination as to the meaning of these many verses that speak about salvation. Instead, we must look to the Fathers of the Church and the Magisterium, in order to show that they support the conclusions which we shall reach.
“For we know that the judgment of God is, according to truth, against them that do such things. And thinkest thou this, O man, that judgest them who do such things and dost the same, that thou shalt escape the judgment of God? Or despisest thou the riches of his goodness and patience and longsuffering? Knowest thou not that the benignity of God leadeth thee to penance? But according to thy hardness and impenitent heart, thou treasurest up to thyself wrath, against the day of wrath and revelation of the just judgment of God: Who will render to every man according to his works. To them indeed who, according to patience in good work, seek glory and honour and incorruption, eternal life: But to them that are contentious and who obey not the truth but give credit to iniquity, wrath and indignation. Tribulation and anguish upon every soul of man that worketh evil: of the Jew first, and also of the Greek. But glory and honour and peace to every one that worketh good: to the Jew first, and also to the Greek. For there is no respect of persons with God. For whosoever have sinned without the law shall perish without the law: and whosoever have sinned in the law shall be judged by the law. For not the hearers of the law are just before God: but the doers of the law shall be justified. For when the Gentiles, who have not the law, do by nature those things that are of the law; these, having not the law, are a law to themselves. Who shew the work of the law written in their hearts, their conscience bearing witness to them: and their thoughts between themselves accusing or also defending one another, In the day when God shall judge the secrets of men by Jesus Christ, according to my gospel. But if thou art called a Jew and restest in the law and makest thy boast of God, And knowest his will and approvest the more profitable things, being instructed by the law: Art confident that thou thyself art a guide of the blind, a light of them that are in darkness, An instructor of the foolish, a teacher of infants, having the form of knowledge and of truth in the law. Thou therefore, that teachest another, teachest not thyself: thou, that preachest that men should not steal, stealest. Thou, that sayest men should not commit adultery, committest adultery: thou, that abhorrest idols, committest sacrilege: Thou, that makest thy boast of the law, by transgression of the law dishonourest God. (For the name of God through you is blasphemed among the Gentiles, as it is written.) Circumcision profiteth indeed, if thou keep the law: but if thou be a transgressor of the law, thy circumcision is made uncircumcision. If then, the uncircumcised keep the justices of the law, shall not this uncircumcision be counted for circumcision? And shall not that which by nature is uncircumcision, if it fulfil the law, judge thee, who by the letter and circumcision art a transgressor of the law? For it is not he is a Jew, who is so outwardly: nor is that circumcision which is outwardly in the flesh. But he is a Jew that is one inwardly and the circumcision is that of the heart, in the spirit not in the letter: whose praise is not of men, but of God. (Romans 2:2-29)
St. Paul makes a number of important claims here:
1.) God will render to every man according to his works. Good to those who do good, and evil to those who do evil.
2.) It is possible for men to do by nature those things that are of the law, and to act according to their conscience, and by fulfilling this law on their hearts, they may be justified.
3.) Being a true Jew is a matter of a circumcision “of the heart”, and is something within. In fact, since St. Paul is referring this to those who have not received the external law, but fulfill the law written on their hearts, this is akin to saying that they are “implicitly” Jews, on account of the interior circumcision of the heart and spirit.
Not only this, but St. Paul even implies that these gentiles have implicit faith in Christ. Why? Because he says that, for some of these men, their thoughts shall defend them, "In the day when God shall judge the secrets of men by Jesus Christ, according to my gospel."
Notice that he says these men shall be judged according to the Gospel. Yet these are gentiles who have not had the truth about God preached to them, but are following the natural law written on their hearts. Hence they do not know explicitly of Christ. Nevertheless, as is abundantly clear in the Scriptures and the Fathers, there is no salvation except through Christ. Hence they must believe in Christ in an implicit manner, since their conscience excuses them when they are judged according to the Gospel.
Also consistent with this explanation, we shall see later in the Fathers that many of them interpret this text as St. Paul explaining how the gentiles before Christ were able to be saved without explicit faith in Him. This reading is certainly consistent with the points made above. It should be noted, however, that St. Paul gives us no indication that his teaching here is limited to the time before Christ, rather the text seems precisely to be talking about a universal state. God will render to every man according to his works, regardless of whether he lived before or after Christ.