Gen 3:15, Gen 12:8, Joel 2:32, Acts 2:21, Rom 10:13, Heb 11:1-2, Heb 11:13

After the death of his second son, Abel, Adam gave birth to a third son, Seth. Seth conveyed to the people the promises of God that he had received from Adam. So from then on people could call on the name of Jehovah. (Genesis 3:15, Gen 4:25-26)

Genesis 3:15 And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.

Gen 4:25 And Adam knew his wife again; and she bare a son, and called his name Seth: For God, said she, hath appointed me another seed instead of Abel, whom Cain slew. 26 And to Seth, to him also there was born a son; and he called his name Enos: then began men to call upon the name of the Lord.

In the Old Testament, when offering sacrifices to God, the name of the Lord was called. Calling on the name of Jehovah was the only way people could be saved. (Genesis 12:8, Joel 2:32)

Genesis 12:8 And he removed from thence unto a mountain on the east of Bethel, and pitched his tent, having Bethel on the west, and Hai on the east: and there he builded an altar unto the Lord, and called upon the name of the Lord.

Joel 2:32 And it shall come to pass, that whosoever shall call on the name of the Lord shall be delivered: for in mount Zion and in Jerusalem shall be deliverance, as the Lord hath said, and in the remnant whom the Lord shall call.

To call on the name of the Lord is to believe and call on Jesus as the Son of God and the Christ. (Acts 2:21, Romans 10:13)

Acts 2:21 And it shall come to pass, that whosoever shall call on the name of the Lord shall be saved.

Romans 10:13 For whosoever shall call upon the name of the Lord shall be saved.

The forefathers of the Old Testament were saved through faith in the coming Christ, the eternal promise of God. (Heb 11:1-2, Heb 11:13)

Heb 11:1 Now faith is the substance of things hoped for, the evidence of things not seen. 2 For by it the elders obtained a good report.

Heb 11:13 These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth.