The adjective means stable, fast, firm. The word in this verse means to be made firm. God confirms us in the faith. That is, God makes us faithful disciples by strengthening us in Christ's provisions. When we come to grasp who and what Jesus Christ is all about, God will establish us in as faithful disciples.
Teaching produces the three characteristics in the previous part of the verse.
Their pastor was faithful in teaching them sound doctrine so that they can have spiritual stability. Faith in God's provisions in Christ is the cement for the building of our faith. That kind of faith can withstand any gale that may come. A strong Christian can withstand the shock of an enemy assault. In this case the assault is the false teaching of the Gnostics.