Walter Marshall (1628-1680) gives sound counsel in The Gospel-Mystery of Sanctification.
I have summarized the main points below.
That we may acceptably perform the duties of holiness and righteousness required in the law, our first work is, to learn the powerful and effectual means whereby we may attain to so great an end.
The scope of all is, to teach you how you may attain to that practice and manner of life which we call holiness, righteousness, or godliness, obedience, true religion….The holiness which I would bring to you is spiritual, Rom vii.14. It consists not only in external works of piety and charity, but in the holy thoughts, imaginations, and affections of the soul, and chiefly in love; from whence all other good works must flow, or else they are not acceptable to God….
The second thing contained in this introductory direction, is the necessity of learning the powerful and effectual means, whereby this great and excellent end may be accomplished….This is a very needful premonition: because many are apt to skip over the lesson concerning the means…as superfluous and useless. When once they know the nature and excellency of the duties of the law, they account nothing wanting but diligent performance; and they rush blindly upon immediate practice, making more haste than good speed.
Several endowments and qualifications are necessary to enable us for the immediate practice of the law. Particularly we must have an inclination and propensity of our hearts thereunto; and therefore we must be well persuaded of our reconciliation with God, and of our future enjoyment of the everlasting heavenly happenings, and of sufficient strength both to will and perform all duties acceptably, until we come to the enjoyment of that happiness.
The way to get holy endowments and qualifications necessary to frame and enable us for the immediate practice of the law, is to receive them out of the fullness of Christ, by fellowship with Him; and that we may have this fellowship, we must be in Christ, and have Christ Himself in us, by a mystical union with Him.
The means or instruments by which the Spirit of God accomplishes our union with Christ, and our fellowship with Him in all holiness, are the gospel, by which Christ enters into our hearts to work faith in us, and faith, by which we actually receive Christ Himself, with all His fullness, into our hearts. And this faith is a grace of the Spirit, by which we heartily believe the gospel and also believe on Christ as He is revealed and freely promised to us in this, for all His salvation.
We cannot attain to the practice of true holiness by any of our endeavours while we continue in our natural state and are not partakers of a new state by union and fellowship with Christ through faith.
Those that endeavour to perform sincere obedience to all the commands of Christ, as the condition by which they are to procure for themselves a right and title to salvation, and a good ground to trust on Him for the same, do seek their salvation by the works of the law, and not by the faith of Christ, as He is revealed in the gospel and they shall never be able to perform sincere and true holy obedience by all such endeavours.
We are not to imagine that our hearts and lives must be changed from sin to holiness in any measure, before we may safely venture to trust on Christ for the sure enjoyment of Himself and His salvation.
Be sure to seek for holiness of heart and life only in its due order, where God has placed it, after union with Christ, justification and the gift of the Holy Ghost and, in that order, seek it earnestly by faith as a very necessary part of your salvation.
We must first receive the comforts of the gospel, that we may be able to sincerely perform the duties of the law.
That we may be prepared by the comforts of the Gospel to perform sincerely the duties of the law, we must get some assurance of our salvation, in that very faith whereby Christ Himself is received into our hearts: therefore we must endeavor to believe on Christ confidently, persuading and assuring ourselves, in the act of believing, that God freely gives to us an interest in Christ and His salvation, according to His gracious promise.
Observe diligently, that the assurance referred to is not a persuasion that we have already received Christ and His salvation, or that we have been already brought into a state of grace; but only, that God is pleased to graciously give Christ and his salvation to us, and to bring us into a state of grace, though we have been altogether in a state of sin and death until this present time….
The assurance referred to is not a persuasion of our salvation, whatever we do, or however we live and walk; but only in a limited way, through mere free grace in Christ, by partaking of holiness as well as forgiveness, and by walking in the way of holiness to the enjoyment of the glory of God. We shall not heartily desire or endeavor to assure ourselves of such a salvation as this is, if we be not brought first to see our own sinfulness and misery, and to despair of our own righteousness and strength, and to hunger and thirst for the sanctifying as well as justifying grace of God in Christ; that so we may walk in the ways of holiness to the enjoyment of heavenly glory.
Beware of thinking so highly of this assurance as if it were inconsistent with any doubting in the same soul.
…The reason why we are to assure ourselves in our faith, that God freely gives Christ and salvation to us particularly, is not, because it is a truth before we believe it, but because it becomes a certain truth when we believe it, and because it will never be true, except we do….
Endeavor diligently to perform the great work of believing on Christ in a right manner, without any delay, and then also to continue and increase in your most holy faith; that so your enjoyment of Christ, union and fellowship with Him, and all holiness by Him, may be begun, continued and increased in you.
Make diligent use of your most holy faith, for the immediate performance of the duties of the law, by walking no longer according to your own natural state, or any principles or means of practice that belong unto it, but only according to that new state which you receive by faith, and the principles and means of practice that properly belong thereunto; and strive to continue and increase in such manner of practice. This is the only way to attain to an acceptable performance of those holy and righteous duties, as far as it is possible in this present life.
Endeavor diligently to make the right use of any means appointed in the word of God, for the obtaining and practicing holiness, only in this way of believing in Christ, and walking in Him, according to your new state by faith.
That you may seek holiness and righteousness, only by believing in Christ and walking in Him by faith, according to the former directions, take encouragement from the great advantages of this way, and the excellent properties of it.
Sanctification in Christ is glorification begun, as glorification is sanctification perfected.