Let’s say a testimony of an individual truth is like a tiny bulb on a Christmas tree. Add another tiny truth, and there’s more light. Add another, still more, and a tree of a hundred tiny 1-watt lights brightens the whole room! Don’t be discouraged if some lights haven’t lit up yet, because in this particular string of lights, just because one is out it doesn’t mean they’re all out, or that the others can’t shine! Even for adults there are always new things to learn and new things to ask the Lord about. I don’t think there is anyone who will know everything there is to know about the gospel in this life.
John Bytheway, “How Do I Know If I Know?”, Deseret Book 2021
A testimony isn't an all or nothing thing. It isn't either on or off. We may believe in one thing, like the existence of God, but struggle to believe in another thing, like maybe the law of tithing.
When we hear that we need to strengthen our testimonies, that means we need to turn on more of the light bulbs on our Christmas tree, or, rather, focus on believing that one thing we are struggling with using faith. And, then, when we have turned that bulb on, we move to the next, and so on, until all the light bulbs are on.
This may be a lifelong process of lighting each bulb. It may even take longer then this life. That doesn't matter. There are things in this gospel that I don't understand completely and even do not agree with, but that doesn't turn off the entire string of light bulbs that are lit up in my life. One day, I will understand and then that bulb will eventually be lit up. Bulb by bulb is the key....line upon line.