Lynda,

In matters like this I go back to the Canons of Dordt. Exactly why God does not change our nature immediately so that we don't sin, the confession does not say. But it does address it in some way. Here are the first three articles of the Fifth Head of the Doctrine:

Quote
Article 1

Whom God calls, according to His purpose, to the communion of His Son, our Lord Jesus Christ, and regenerates by the Holy Spirit, He delivers also from the dominion and slavery of sin in this life; though not altogether from the body of sin and from the infirmities of the flesh, so long as they continue in this world.

Article 2

Hence spring daily sins of infirmity, and hence spots adhere to the best works of the saints, which furnish them with constant matter for humiliation before God, and flying for refuge to Christ crucified; for mortifying the flesh more and more by the spirit of prayer, and by holy exercises of piety; and for pressing forward to the goal of perfection, till being at length delivered from this body of death, they are brought to reign with the Lamb of God in heaven.

Article 3

By reason of these remains of indwelling sin, and the temptations of sin and of the world, those who are converted could not persevere in a state of grace if left to their own strength. But God is faithful, who having conferred grace, mercifully confirms and powerfully preserves them therein, even to the end.

I don't know if you are familiar with the Canons of Dordt. You can find it here
on the Highway. It really is worth reading and studying.

Johan