In order for the TX/RX and CTS/RTS pins on the MAX3100 to operate correctly, a valid oscillator must be present. The X1 and X2 pins have built-in oscillator drivers, which allows for a variety of crystals to be placed between those two pins. Proper load capacitance values are required to sufficiently load the crystal. Additionally, an external clock source could be applied to X1 with X2 unconnected.
Since the SPI interface operates independently of the clock applied to the MAX3100, the internal registers can be written and read back regardless of the presence of an oscillator. This doesn't guarantee that the UART interface is operational.