Try the Spirits / Prove All Things

1 John 4:1 Beloved, believe not every spirit, but try (#1381) the spirits whether they are of God: because many false prophets are gone out into the world.

Compared to:

1 Thessalonians 5:21 Prove (#1381) all things; hold fast that which is good.

We try (prove) the spirits (the teachings from the Bible we hear), and in doing so, we "prove all things" doctrinally, and then "hold fast" on those things which are true and faithful (good).

In other words, a faithful hearer of the Word will be actively checking out the things he/she hears taught to make sure they are truthful and correct. And, once "all things" are verified in this way, the doctrine may be held onto.