In a Nutshell:
To say “Islam means submission to one God” says little about Islam as saying "secularism is the ideology that upholds separation of church and state". While true, it does not describe how such a separation is made possible, enacted, and reenacted.
Islam is a 'deen' which translates to a "way of life" or a "discursive tradition intimately fused with power" where Muslims are expected to socially and territorially embody Islam (dar al-Islam) - acknowledging, submitting, and serving Allah. In other words it is reified revelation, an objective reality. Islam thus comprises an environment derived from the fusion of revelation with power, where every individual chooses their way of life and freely lives by it.
The first concrete manifestation of Islam was the Prophet's Medinah followed by the domains of the early Khulafah after him, the Umayyads, the Abbasids and the Ottomans, with Islam being dismantled in 1924 by European powers resulting in the absence of Islam for a century.