Even the Bible admonishes us to test to see whether we are in the faith:
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Keep testing whether you are in the faith; keep proving what you yourselves are. Or do you not recognize that Jesus Christ is in union with you? Unless you are disapproved."-2 Corinthians 13:5.
Of course these words are directed to the Christian congregation. But how can you be sure which of all the Christian religions is the correct one? That would take some testing. Are all of them? Many people don't even believe in the Bible or in the Christ. Does that mean it is false? So like you said there is a lot of testing to be done.
For a person that doesn't believe in God, first they need to be given convincing proof of his existence. Many sound-minded and intelligent scientists after studying their fields of science come to a logical conclusion that there has to be an intelligence behind the great forces of the universe, nature, and the wonders of the living world.
Archeological records show that the Bible is accurate in regards to history. In fact many times it accuracy has only been proven after many centuries of criticism. And its faithful prophecies of future events shows us that it is really God's word.
https://www.jw.org/en/library/magazines/watchtower-study-october-2021/Hold-Fast-to-the-Truth-With-Strong-Conviction/
If you are interested you may want to check out this recent study article I actually just finished studying it a couple of days ago and it immediately came to mind when I read your question. It starts by propounding the question that how can we be sure we have the truth out of the tens of thousands of so-called Christian denominations.
Some very convincing proofs were given about having the truth: JWs reject idolatry. Idolatry in all its forms. Even the worship of humans. In many charismatic movements humans are revered and their church leaders promote their own self-styled books and people donate much money to them. Yet Jesus told his followers that they are all brothers. (Matthew 23:8-10). The fact that there is no laity and clergy distinction among Jehovah's people as it was in the Bible is a strong convincing proof.
Another great proof is the respect Jehovah's Witnesses have for Jehovah's name. Jesus prayed: "Father glorify your name." And thorughout his earthly ministry Jesus said he glorified God's name. (John 12:28; John 17:26.)
And in Acts 15:14 we are told: "Symʹe·on has related thoroughly how
God for the first time turned his attention to the nations to take out of them a people for his name." What group of Christians today have the honor, the distinction of being given Jehovah's name as his witnesses? The Bible said this happened.
In stark contrast to Jehovah's Witnesses who love the name of God and uphold it and God's high moral values found in the Bible many Church leaders throughout history have done everything in their power to hide that name. Many have removed it entirely from their Bible translations substituting God's name Jehovah with the titles "LORD" or "GOD."
We love the truth. The Bible calls pure worship "the way of truth."-2 Peter 2:2. In the first century Christians rejected religious ideas, cultural traditions, and personal opinions that were not in harmony with this way of truth. Just as the first century Christians and even Bible writers such as Paul did not have full understanding of all things Jehovah's Witnesses do not claim to understand all things in the Bible. And as they study it and discern a certain practice or teaching is out of line with God's words they change to better adhere to the truth. Imagine if other "Christian" religions were as humble and as eager and ready to change to please God!
And the greatest convincing proof of course is the love God's people have for each other. Jesus said that all the world would know his true followers because they would have love among each other.-John 13:34-35.
The Christian scripture tells us there is only one faith. And to test to see whether you are in the faith. There is one baptism, and God is a God of peace not of disorder. It explains the structural arrangement of the Christian congregation that there are traveling overseers, elders, and ministerial servants. As well, instead of trying to change to appease the feelings or sentiments of the day or the people who live immorally degraded lives, they uphold the sanctity of marriage, and the sacredness of blood, and keep the congregation clean by expelling unrepentant wrongdoers. All based on patterns found in the Bible. (Acts 15:28-29; 1 Corinthians 5:11-13; 1 Corinthians 6:9-10; Hebrews 13:4.
Jesus said that many would claim to be his followers but not all of them are genuine. And that many in the end would have an appearance of godliness but prove false to its power. And to reiterate the point it clearly states that there is only "one faith" "one baptism":
"One body there is, and one spirit, just as you were called to the one hope of your calling; 5 one Lord, one faith, one baptism; 6 one God and Father of all, who is over all and through all and in all."-Ephesians 4:4-6.
It is very important to test whether you are in the faith!
