Ignatius once again encourages them to avoid false doctrine, but here specifically he is referring to Judaism.
"Be not deceived with strange doctrines, nor with old fables, which are unprofitable. For if we still live according to the Jewish law, we acknowledge that we have not received grace. For the divinest prophets lived according to Christ Jesus. On this account also they were persecuted, being inspired by His grace to fully convince the unbelieving that there is one God, who has manifested Himself by Jesus Christ His Son, who is His eternal Word, not proceeding forth from silence, and who in all things pleased Him that sent Him." (Ignatius to the Magnesians, Chapter 8)The reference to "proceeding from the silence" could very well be in reference to the teachings of Valentinus who's Aeon Sige (or silence) brought forth their christ. He reminds us that we cannot embrace Jewish law and Christian grace at the same time. Either we embrace the law and live by the things in it or we embrace Jesus and live by His grace. It is as Paul said of those who sought to live by the law after coming to Christ, "You have been severed from Christ, you who are seeking to be justified by law; you have fallen from grace." (Galatians 5:4) God has called us all to Himself and had called the Jews to leave behind the law, which no man can keep and which never made anyone righteous, and to receive His grace which is able to cleanse and heal us and to give us an inheritance in the Kingdom of God.
"If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord’s Day, on which also our life has sprung up again by Him and by His death... how shall we be able to live apart from Him, whose disciples the prophets themselves in the Spirit did wait for Him as their Teacher? And therefore He whom they rightly waited for, being come, raised them from the dead." (Ignatius to the Magnesians, Chapter 9)
The law was for a time only, as Jesus said, "For all the prophets and the Law prophesied until John." (Matthew 11:13) but His grace is for all times. God gave His law for a time until something greater would come; until a time of reformation. "Accordingly both gifts and sacrifices are offered which cannot make the worshiper perfect in conscience, since they relate only to food and drink and various washings, regulations for the body imposed until a time of reformation." (Hebrews 9:9-10) We are now in that time of reformation, now is the time to embrace His grace.
"Let us not, therefore, be insensible to His kindness. For were He to reward us according to our works, we should cease to be. Therefore, having become His disciples, let us learn to live according to the principles of Christianity. For whosoever is called by any other name besides this, is not of God. Lay aside, therefore, the evil, the old, the sour leaven, and be ye changed into the new leaven, which is Jesus Christ. Be ye salted in Him, lest any one among you should be corrupted, since by your savour ye shall be convicted. It is absurd to profess Christ Jesus, and to Judaize. For Christianity did not embrace Judaism, but Judaism Christianity, that so every tongue which believeth might be gathered together to God." (Ignatius to the Magnesians, Chapter 10)
When Ignatius write of the "principles of Christianity" he is not writing of the "principles of Christians" but of the "principles of Christ." We must not see Christianity as pertaining to a group of people, where we are called to become like others in that group, but rather as being a devotion to Christ, where we are called to become like Him. The word of reconciliation that proceeds from our mouths should not be, "Come and be like us," rather it should be "Come and be like Him." This is true Christianity.