Does it mean it has existed forever and would exist forever? Does it mean it is changeless? The evidence shows that India is now only 80 percent Hindu and there are all kinds of forces trying to reduce this number. How does this square with Hinduism being "Sanatana Dharma"?
Sanatana (सनातन) is a Sanskrit word referring to eternal, having no beginning or end. The phrase dharma sanātana does occur in Hinduism scriptures. Traced back as far as valmiki Ramayana . The word is also present in Bhagvat Purana and in Manu smriti.
कृते च प्रतिकर्तव्यमेष धर्मः सनातनः।
सोऽयं त्वत्प्रतिकारार्थी त्वत्तः सम्मानमर्हति ।। VR 5.1.114VR 5.1.114।।
कृते when help is rendered, प्रतिकर्तव्यम् it should be repaid, एषः this, सनातनः eternal, धर्मः dharma, तत्प्रतिकारार्थी for doing service in return, सः अयम् this, त्वत्तः from you, सम्मानम् honour, अर्हति he deserves.`
When a service is done, a return service needs to be rendered. This is an ancient tradition. Such this ocean who wants to do a return service to the Raghu dynasty, is suitable for respect by you."
चतुर्युगान्ते कालेन ग्रस्तान्छ्रुतिगणान्यथा ।
तपसा ऋषयोऽपश्यन्यतो धर्मः सनातनः ॥ SB 8.14.4॥
catur-yugānte kālena grastāñ chruti-gaṇān yathā
tapasā ṛṣayo ’paśyan yato dharmaḥ sanātanaḥ
At the end of every four yugas, the great saintly persons, upon seeing that the eternal occupational duties of mankind have been misused, reestablish the principles of religion.
सत्यं ब्रूयात् प्रियं ब्रूयान्न ब्रूयात् सत्यमप्रियम् ।
प्रियं च नानृतं ब्रूयादेष धर्मः सनातनः ॥ 4.138 ॥
satyaṃ brūyāt priyaṃ brūyānna brūyāt satyamapriyam |
priyaṃ ca nānṛtaṃ brūyādeṣa dharmaḥ sanātanaḥ || 138 ||
He shall say what is true; and he shall say what is agreeable; he shall not say what is true, but disagreeale; nor shall he say what is agreeable, but untrue; this is the eternal law.—(138)
So I have traced the word uptill Valmiki Ramayana .