Never had a doubt about who has the truth.
If you have never had such doubts, what is it that has kept you sure in your faith? Have you ever looked into other faiths, not necessarily for conversion, but maybe just out of interest?
Study, prayer, meditation, reflection. I was taught basic Bible truths when I was a kid such as the soul dies at death. There is one God named Jehovah. His son is Jesus Christ. I was given convincing proof that the Bible is God's word. Studied archeology, history, science, and other facts that all agree with the Bible. And all of the prophecies in the past that have faithfully come true, and see many prophecies that are now being fulfilled.
The words "let God be true yet every man a liar" always ring true in my mind when I see teachings in religions all around me that do not agree with the Bible. I see people that claim to worship the God of the Bible worship the cross, believe in a Hellfire, participate in war, preach a false Trinity. My Bible education showed me all of this was false when I was a little kid. And I know the Bible is true, so those religions don't have truth.
All religions teach that the soul is immortal. The Bible teaches the soul is mortal and dies at death. So it is easy to discard any religion that teaches that we have an inherently immortal soul. Ezekiel 18:4.
I have no doubt in the existence of the one true God Jehovah. That his son gave his life for the sins of mankind so that exercising faith in him we might have everlasting life. I have talked with people of all faiths and creeds over the years, and those with no faith. I have experiences the supernatural. I've fought with demonic entities. I've witnessed angelic protection, and direct answers to my prayers. When I was in despair I saw the loving hand of Jehovah care for me and carry me through.
The closer we move toward the end of this system of things in Satan's hands the more I see Jehovah's protective hands operative among his people and in my life. The more I see with my eyes his love and his tender care and his mercies upon his people.
Jehovah's Witnesses preach that God's kingdom is the only solution to mankind's problems. They do not get involved in politics and war. Jesus said his people were no part of the world, and that they would love each other.
World governments use the threat of death to convince people to do horrible things and to betray their faith. But God gives us the hope of a resurrection if we die. He thus removes one of the strongest weapons Satan has at his disposal, the threat of death. By means of that hope Jehovah's Witnesses remained faithful to God when under brutal Nazi oppression, and died with integrity intact in the concentration camps, refusing to Heil Hitler and fight for the Nazi whom the Pope of the Catholic Church decreed was God's messenger on earth.
I have come across many who have tried to erode my faith in the Bible over the years. But none have had convincing proofs. And none have replaced it with anything better.