Idolatry is making a good thing into an ultimate thing. Talk about the Lordship of Jesus, the supremacy of Jesus, without honoring him with the firstfruits of your labor, is just that: talk.
“We will lay anything at his feet conceptually – except for our hidden idol. Tithing is one of the few ways to really mortify the idol of consumerism and materialism.” Douglas Wilson (Hebrews Commentary)