Because pleasing Kṛṣṇa has become our ultimate goal, all our activities have become spiritual activities.
(Message of Godhead).

It is explained in the Bhagavad-gītā that through performance of work with transcendental results, everything becomes spiritualized.
(Message of Godhead).

At the final stage, flowers, fruits, plants, trees, clay—all objects and elements—become spiritualized by being used in Lord Kṛṣṇa's service. In other words, nothing is seen to be separate from the Lord.
(Renunciation Through Wisdom).

Once everything in the world is conducted according to the Lord's desire, then all activities will become spiritual and the Lord's presence will be felt everywhere and in everything.
(Renunciation Through Wisdom).

Since the Supreme Personality of Godhead is the original cause of everything, the Vaiṣṇava sees everything in relationship with Kṛṣṇa, even in this material world. By such advanced knowledge, everything becomes spiritualized. In other words, everything in the material world is already spiritual, but due to our lack of knowledge we see things as material.
(Krsna Book).

Even the very body of a devotee becomes spiritualized, just as an iron rod put into fire becomes as qualified as fire because it becomes red hot and will immediately burn anything it touches.
(Sri Caitanya Caritamrta----3:5:119----purport).

Having attained knowledge and the spiritual platform, the devotee always engages in the service of the spiritual body of Kṛṣṇa. When one is freed from material connections in this way, his body immediately becomes spiritual, and Kṛṣṇa accepts his service.
(Sri Caitanya Caritamrta----3:4:194---purport).

Let us therefore offer our obeisances unto the Supreme Personality of Godhead, by whose mercy so-called material things also become spiritual when they are engaged in spiritual activity.
(Srimad Bhagavatam----8:3:2---purport).

When one is liberated, one's sva-dharma—specific duty—becomes spiritual and is not in the material bodily concept.
(Bhagavad-Gita----2:31---purport).

The more the activities of the material world are performed in Kṛṣṇa consciousness, or for Viṣṇu only, the more the atmosphere becomes spiritualized by complete absorption.
(Bhagavad-Gita----4:24---purport).

When the mind, intelligence and ego are purified, all the active senses of the living entity become spiritual.
(Srimad Bhagavatam---4:29:61---purport).