35 Let us therefore make use of this great benefit, and seek for personal experience of this most needful Gift. For the Apostle says, in words I have already cited, But we have not received the spirit of this world, but the Spirit which is of God, that we may know the things that are given unto us by God. We receive Him, then, that we may know. Faculties of the human body, if denied their exercise, will lie dormant. The eye without light, natural or artificial, cannot fulfil its office; the ear will be ignorant of its function unless some voice or sound be heard; the nostrils unconscious of their purpose unless some scent be breathed.
Not that the faculty will be absent, because it is never called into use, but that there will be no experience of its existence. So, too, the soul of man, unless through faith it have appropriated the gift of the Spirit, will have the innate faculty of apprehending God, but be destitute of the light of knowledge. That Gift, which is in Christ, is One, yet offered, and offered fully, to all; denied to none, and given to each according to the measure of his willingness to receive; its stores the richer, the more earnest the desire to earn them.
This gift is with us unto the end of the world, the solace of our waiting, the assurance, by the favours which He bestows, of the hope that shall be ours, the light of our minds, the sun of our souls. This Holy Spirit we must seek and must earn, and then hold fast by faith and obedience to the commands of God.
Source: On the Trinity (New Advent)