You can’t make a delicious cake if the ingredients are bad. And the building materials have to be top quality to build a lasting edifice. In the same way, in the spiritual realm, in order to produce in the human life the things that please God, one has to start with true teaching:

But the aim of our instruction is love that comes from a pure heart, a good conscience, and a sincere faith.
1 Timothy 1:5 NET

Paul had just ordered Timothy, at the beginning of the letter, to make some people stop teaching false doctrine, for it’s impossible to do God’s work in a person’s life when one begins with wrong teachings.

It’s common to say that what matters is what’s in the heart. That’s true, as far as it goes.

In order to have a pure heart that produces the love of God, not to mention a good conscience and a sincere faith, we have to start with sound doctrine. With the truth of the gospel. With right teaching.

Without that, all the rest is spoiled.