Does love believe everything, or should we test what we are told?

In 1 Corinthians 13 Paul is describing how love treats people, not how the mind should weigh claims. Every line of the passage is about our dealings with others: love is patient and kind, is not easily provoked, and “thinketh no evil” (13:4–5). So ‘believeth all things’ means that love is ready to believe the best of others. It does not assume bad motives, and it gives people the benefit of the doubt. Barnes says that the whole argument requires us to read the words of the conduct of others, and that they cannot describe a love that believes anything at all. Gill adds that such a person still will not believe every spirit or every teacher. The same passage says that love “rejoiceth in the truth” (13:6), so it does not stop caring what is true.

The other passages are about something else: accepting what we are told, whether by a flatterer or by someone who claims to speak for God. Here Scripture asks for care. The prudent man “looketh well to his going” (Proverbs 14:15), and believers are to “try the spirits whether they are of God”, because “many false prophets are gone out into the world” (1 John 4:1). Paul, who wrote 1 Corinthians 13, also wrote “test all things” (1 Thessalonians 5:21, in the BSB).

In practice the two belong together. A Christian can be generous towards people and slow to think evil of them, and still be careful about the claims and teachings he is asked to accept.