There are many different ways of securing FPGAs to prevent successful reverse engineering. One of the common forms is Obfuscation methods. In this paper, we proposed an approach based on Obfuscation to prevent FPGAs from successful reverse engineering and, as a result, hardware trojan horses (HTHs) insertion. Our Obfuscation method is using configurable look up tables (CFGLUTs). We suggest to insert CFGLUTs randomly or based on some optional parameters in the design. In this way, some parts of the design are on a secure memory, which contains the bitstream of the CFGLUTs so that the attacker does not have any access to it. We program the CFGLUTs in run-time to complete the bitstream of the FPGA and functionality of the design. If an attacker can reverse engineer the bitstream of the FPGA, he cannot detect the design because some part of it is composed of CFGLUTs, which their bitstream is on a secure memory. The first article uses CFGLUTs for securing FPGAs against HTHs insertion, which are results of reverse engineering. Our methods do not have any power and hardware overhead but 32 clock cycles time overhead.