Is it possible to be a worshiper of the true God without being reached or evangelized?
The short answer is yes.
God is able to be believed in through the very aspect of nature of thing on earth (nature) and contemplation of our universe, albeit in a limited manner.
St. Thomas Aquinas explains this in five steps.
The Five Ways
I answer that, The existence of God can be proved in five ways.
The Argument from Motion
The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from to . But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.
The Argument from First Cause
The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away
the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
The Argument from Necessity
The third way is taken from possibility and , and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, be- cause that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence—which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the exis- tence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.
The Argument from Gradation
The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But “more” and “less” are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.
The Argument from Design
The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God. - From the Nature of the Universe” by Thomas Aquinas
Recall that St. Paul in the Areopagus, stated that some pagans worshiped the true God without knowing it!
In Acts 17, Paul arrives in Athens, the citadel of the many Greek gods. In that city was the Areopagus, or Mars Hill, where a council of civic leaders met. This council, also called the Areopagus, had charge of religious and educational matters in Athens.
While in Athens, Paul was provoked by the many idols he saw. As was his custom, he went to the synagogue and reasoned with the Jews and God-fearing Gentiles. He also preached to those in the marketplace. That is when he encountered the Epicurean and Stoic philosophers, who were always looking to discover something “new” to discuss. The Epicureans were followers of Epicurus (341—270 BC), who taught that happiness was the ultimate goal in life. The Stoic thinkers regarded Zeno (340—265 BC) as their founder. He was noted for promoting the rational over the emotional. Both Epicurus and Zeno believed in many gods.
Hearing Paul teach about Jesus, the philosophers had Paul come to the Areopagus and asked him to tell them about this “new,” strange teaching he was proclaiming. Standing in the midst of the Areopagus, Paul tells those gathered that he realized Athenians were very religious, having seen their many objects of worship. But one altar among the many caught his attention. On it were inscribed the words “TO AN UNKNOWN GOD.” In their ignorance, the Greeks had erected an altar to whatever god they might have inadvertently left out of their pantheon. Paul masterfully uses this altar as an opportunity to share the one true God.
Since the Greeks obviously didn’t know who this god was, Paul explains that this “unknown god” was the biblical God, the Creator of heaven and earth, who does not dwell in temples made with hands. Actually, God is the Source of life for all nations, and He is really the One they were unwittingly seeking. Paul says God is near; in fact, “in him we live and move and have our being” (Acts 17:27–28). The Greeks, however, were unable to find the true God on their own, so God came searching for them. He calls all men to repent and accept Jesus Christ, who was raised from the dead and will judge the world in righteousness.
Paul’s mention of the resurrection brought a varied response from the philosophers. Some sneered outright. Others said they wanted to hear more from Paul (Acts 17:32). Praise the Lord, some believed. One of the members of the Areopagus, named Dionysius, exercised faith in Christ, and several other Athenians also became Christians that day.
The “unknown God” desires to be known. That is why He has spoken to us through His Word; that is why He sent His Son into the world (Luke 10:22). God can be known through faith in Jesus Christ. Jesus said, “Anyone who has seen me has seen the Father” (John 14:9).
The search for God will always be there in the hearts of those who genuinely look for the truth, with the situation they actually find themselves in and live according to the natural law!