Jainism did not start with Bhagwan Rishabhdev (Ādināth).

In Jain dharma:

• Jain dharma is anādi-anant – beginningless and endless.
• Rishabhdev Bhagwan is the first Tirthankar of the present time-cycle (current avasarpinī), not the “founder” of Jainism.
• Before him, in the infinite past, there have been countless Tirthankars in previous time-cycles, all teaching the same eternal Jain dharma (ahiṁsā, aparigraha, satya, brahmacharya, anekānt, etc.).

What changes is:

• The world cycle (kālchakra) and the names and number of Tirthankars in each half-cycle.
• Sometimes dharma becomes almost forgotten among people; then a Tirthankar appears and re-establishes it clearly again.

So, in simple words:

• Jainism existed eternally as truth and right conduct.
• Rishabhdevji is the first Tirthankar of our current cycle who re-taught this eternal Jain dharma to beings of this era.
• He is not the “start” of Jainism; he is the first teacher of this era of an eternal religion.