Whetting your thoughts = faith

"Faith is not a sleeping thing, nor merely not doubting that Christ is mine, but a continual active whetting of my thoughts on Christ as mine ..." - Thomas Goodwin

Faith is not the absence of doubt. It is not the negation of incertitude or uncertainty. It is active and participatory. It is a 'sharpening through friction' of our thinking. It is a 'stimulation and making keen' of this thought: Christ is mine. You do not 'whet' your thoughts and thinking passively. You must be proactive and purposeful. You must read. You must think. You must converse.

Due estimate

Thomas Goodwin on a Christians highest privilege and greatest interest, namely, patience and suffering:

If you are true and right Christians, and you know,as the apostle says, how to put a due estimate on what is your greatest interest and privilege in this life-

the proof and trial of your graces, and of the grace of patience above all, as the highest perfection of a Christian, yea, of Christ Himself, and the most eminent praise of prophets and apostles -

and if you value being rendered most pleasing to God, then count it all joy when you fall into temptations.

For now you have God and Christ, the great, the Chief Master Orderer and Designer of these conflicts, setting His most gracious eye on you, pleasing Himself to behold how valiantly, wisely, and gallantly you behave and acquit yourselves. (61, line breaks mine)

The Spirit fires our hearts

From Thomas Goodwin's Exposition of Ephesians as relayed in A Habitual Sight of Him edited and introduced by Joel R. Beeke and Mark Jones:

You see, the heart is made the subject of Christ’s dwelling; it is not to dwell in the notion, in your brains. You have no indwelling of Christ unless your hearts are affected.

This is expressly said:
“We all, beholding with open face the glory of the Lord, are changed into the same image from glory to glory, by the Spirit of the Lord” (2 Cor. 3118).

First, there is spiritual beholding, he speaks of Christ, not only of His person, but of all that is to be known of Christ. Adam’s graces had no glory, but all of Christ’s do.

Such a beholding, letting Christ and His glory into the soul, changes it, turns it, and leaves impressions on it. This is done by the Spirit of the Lord. By “the Spirit of the Lord” the apostle does not mean the Holy Ghost, for he said in the words before, “The Lord is that Spirit.”

It is Christ Himself, by His force and power when He comes into the soul, who changes it, fills it, quickens it, strengthens it, and leaves impressions on it.

As the burning glass contracts all the beams of the sun to a point, but it is the beams of the sun that set on fire the cloth, so it is the Spirit of the Lord that fires our hearts. Thus, to know Christ is to dwell in Him. (57, line breaks mine)

What is it to have Christ dwell in the heart by faith?...
First, it is to have Jesus Christ continually in one's eye, a habitual sight of Him ... you should carry along and bear along in your eye the sight and knowledge of Christ, so that at least a presence of Him accompanies you... (55)