A Christian is a Disciple of Christ. As disciple, he receives the holy spirit, like Jesus Christ did (Lu 4:18), to continue the work that his Lord had started (Jo 20:21), the work of witnessing (Ac 1:8) to the truth. A disciple who fulfills this obligation (Ec 12:13) becomes a LIGHT or a SALT to the world.

How and when does a disciple become a LIGHT of the world (Mat 5:14)?

A disciple whose good works are done in the midst of the ones who do not know what is good, becomes a LIGHT to these ones.

Therefore, he comes out (Mat 5:15) to the world, and through the spirit (Eph 2:13), proclaims the truth, which is his good works (Eph 2:10; 5:9), that shines to the world (Php 2:15), to those who are coming to the light (Joh 3:21), seeking the way of truth.

He is the lead and guide, so that those coming to the light follow (Jer 50:8).

How and when does a disciple become a SALT of the earth (Mat 5:13)?

A disciple who persists in his good works, in the midst of those not wanting to come to the light but continue with their wicked ways (Joh 3:20), becomes SALT to these ones.

Therefore, he persists (2Cor 6:17) to keep himself unspotted from the hypocritical faith of the world (Jas 1:27) and continues his good works of proclaiming the truth, in the midst of the world that rejects it, even when he is hated, reproached, excluded, rejected, and persecuted (Lu 6:22-23).

He is different, so the world counts him last (Mr 10:31), but his works serve to reprove and remind those rejecting the light, of what is good and pleasing to God (Eph 5:11).