Low-mass galaxies in the early universe are believed to be the building blocks of present-day galaxies. These fledgling systems likely played a pivotal role in cosmic reionization, a major phase transition from neutral Hydrogen to ionized plasma around 600-800 Myr after the Big Bang. However, these galaxies have eluded comprehensive spectroscopic studies owing to their extreme faintness. Here we report the first spectroscopic analysis of 8 ultra-faint galaxies during the epoch of reionization with absolute magnitudes between M_UV ∼ -17 to -15 mag (down to 0.005 L^⋆). The combination of ultra-deep NIRSpec (Near-Infrared Spectrograph) observations and the strong gravitational lensing boost of Abell~2744 allow us to derive the first spectroscopic constraints on the prevalence of faint galaxies and their ionizing properties during the Universe’s first billion years. We find that faint galaxies are prodigious producers of ionizing photons with log(ξ_ion/ Hz erg^-1) =25.8± 0.05, a factor of 4 larger than canonical values. This means that the total rate of ionizing photons produced by galaxies exceeds that needed for reionization, even for modest values of escape fraction (f_esc =5%). These findings provide robust evidence that faint galaxies were the main drivers of cosmic reionization at z∼7.