But what is the immediate goal you must earnestly ask, for if it is not in the same way discovered by us, we shall strive and wear ourselves out to no purpose.
[...] The end of our profession…is the kingdom of God or the kingdom of heaven.
But the immediate aim or goal is purity of heart, without which no one can gain that end. Fixing our gaze then steadily on this goal, as if on a definite mark, let us direct our course as straight towards it as possible.
And, if our thoughts wander somewhat from this, let us revert to our gaze upon it, and check them accurately as by a sure standard.
This will always bring back all our efforts to this one mark, and will show at once if our mind has wandered ever so little from the direction marked out for it.
[...] The end which we have set before us is, as St Paul says, eternal life, as he declares, “having indeed your fruit unto holiness, and the end eternal life.”
But the immediate goal is purity of heart, which he not unfairly terms “sanctification,” without which the afore-mentioned end cannot be gained.
It is as if he had said in other words: “having your immediate goal in purity of heart, but the end life eternal.”
Of this goal the same blessed Apostle teaches us, and significantly uses the very term, i.e., [in Greek] skopos, saying as follows:
“Forgetting those things which are behind and reaching forward to those that are before, I press toward the mark, for the prize of the high calling of the Lord.”
This is more clearly put in Greek… “I press toward the mark,” as if he said, “With this aim, with which I forget those things that are behind, i.e., the faults of earlier life, I strive to reach as the end the heavenly prize.”
Whatever then can help to guide us to this object; viz., purity of heart, we must follow with all our might, but whatever hinders us from it, we must shun as a dangerous and hurtful thing.
[...] For the mind, which has no fixed point to which it may return, and on which it may chiefly fasten, is sure to rove about from hour to hour and minute to minute in all sorts of wandering thoughts, and from those things which come to it from outside, to be constantly changed into that state which first offers itself to it.
John Cassian (c. 360-435): Conferences 1,4-5.