God made all men in his own image.
Each person had the capacity to follow God, and not knowing good or evil had nothing to do with it. But when we chose to be God when we wanted to be in his place, our hearts instinctively understood what was good and what was evil. Anything robbing God of His status in our lives is sin. The act of rebellion against God demonstrates that we know how to do good and how to do evil.
Whether or not we know God, we know who he is. We know by "his eternal power and divine nature...Being understood from what has been made, so that men are without excuse"(Romans 1). All mankind knows what is good and evil. Everyone has a conscious capable of understanding the Lord's rightful position in their life. Everyone knows that they are missing something. The lack of fulfillment is why everyone, the great and small, constantly seek a way to fill the void. Some seek money. Some seek power, intelligence, fame, or fortune. Some set up religions with intricate requirements to make it to God. But God said otherwise. God shows us by his perfect and just nature that we aren't God and have no power nor authority to reach his glory. But God loves us because he made us and put his image in us. We cannot reach God, but, praise Jesus, God seeking us.
God knows what is good and what is not. We, being his sons and daughters, are being taught what is good and holy and pleasing. We ought not debate this matter with such generality, because each and every person is capable of receiving the Son, at any moment, anywhere. If anynone will seek after the truth in earnest, the Truth, a who not a what, will seek them. Christ, who is the way the truth and the life, will then run to them, and show them through the Holy Ghost, who is their conscience.
Each man and woman can except God. False gods, idolatry, and selfishness are conscious decisions to REJECT God. Our job is to help people realize that they are making things to hard for themselves.
As for "saints" in religions of the world. Though they are not Sons and Daughters of God in Christ Jesus, the Lord is still able to use Satan's deceptions to do good to people. All good things come from God, and nothing good comes from Satan.
We must not scrutenize them, but ourselves. We must follow Christ so that we may prove by our lives that our Savior lives, and that he is the only way to the Father.
So God made it simple. That is you believe in your heart that Jesus Christ is Lord, and your Lord, and you confess with your mouth that Jesus Christ is Lord, you will be saved.
And that's all I have to say about that.