You are working off the premise that the altar is somehow defiled because it is used in a Sunday church. The same God manifests Himself in the Sunday church, He is the same God we worship. His Holy Spirit is active in the Sunday churches too, and He blesses His children there. The only difference between them and us is that we have more knowledge. Other than that, our God is the exact same. Jesus said, "And other sheep I have which are not of this fold; them also I must bring, and they will hear My voice; and there will be one flock and one shepherd." (John 10:16). "My sheep hear My voice, and I know them, and they follow Me." (John 10:27). "No one can come to Me unless the Father who sent Me draws them" (John 6:44). The Bible tells us the Spirit "will guide you into all truth" (John 16:13), and perhaps God will lead them into knowledge of the Sabbath in the future. The soul who seeks God in sincerity will not be turned away from God's Spirit, for we are told: "love covers all sins" (Proverbs 10:12). We are also told "he who turns a sinner from the error of his way will save a soul from death and cover a multitude of sins" (James 5:19). Maybe it is your calling to tell the Sunday church congregation the leader about the Sabbath. And it will take time for them to ponder the issue before they take a stand for the truth.